diff --git a/bin/cnc25d_example_generator.py b/bin/cnc25d_example_generator.py
index 1c2d68e..b2971a3 100755
--- a/bin/cnc25d_example_generator.py
+++ b/bin/cnc25d_example_generator.py
@@ -33,6 +33,8 @@
 
 import os, re, sys
 import subprocess
+import six
+from six.moves import input
 
 ##############################################################################
 # Checking FreeCAD installation
@@ -76,11 +78,11 @@
 freecad_verion_major = int(freecad_verion[0])
 freecad_verion_minor = int(freecad_verion[1])
 if((freecad_verion_major==0)and(freecad_verion_minor<13)):
-  print("ERR056: Error, Your FreeCAD version is too old! You have {:d}.{:d} and 0.13 or newer is needed.".format(freecad_verion_major, freecad_verion_minor))
+  six.print_(("ERR056: Error, Your FreeCAD version is too old! You have {:d}.{:d} and 0.13 or newer is needed.".format(freecad_verion_major, freecad_verion_minor)))
   print("Please, install the latest FreeCAD version on your system and re-run this script.")
   sys.exit(1)
 #info
-print("The FreeCAD version {:d}.{:d} is installed on your system.".format(freecad_verion_major, freecad_verion_minor))
+six.print_(("The FreeCAD version {:d}.{:d} is installed on your system.".format(freecad_verion_major, freecad_verion_minor)))
 
 ### check if the FreeCAD Library can be imported
 
@@ -4014,8 +4016,8 @@ def sub_canvas_graphics(ai_rotation_direction, ai_angle_position):
 ceg_example_list_sorted_keys = sorted(ceg_example_list.keys())
 print("\nThis executable helps you to generate the following cnc25d script examples in the current directory:")
 for l_example in ceg_example_list_sorted_keys:
-  print("  +  {:s}".format(l_example))
-user_choice=raw_input("Do you want to generate all these upper files in the current directory? [yes/No] ")
+  six.print_(("  +  {:s}".format(l_example)))
+user_choice=input("Do you want to generate all these upper files in the current directory? [yes/No] ")
 if((user_choice=='yes')or(user_choice=='y')):
   for l_example in ceg_example_list_sorted_keys:
     fh_output = open(l_example, 'w')
@@ -4025,14 +4027,14 @@ def sub_canvas_graphics(ai_rotation_direction, ai_angle_position):
 else:
   print("Choose which cnc25d script example you want to generate in the current directory:")
   for l_example in ceg_example_list_sorted_keys:
-    print("cnc25d script example : {:s}".format(l_example))
-    user_choice=raw_input("Do you want to generate the file {:s} in the current directory? [yes/No] ".format(l_example))
+    six.print_(("cnc25d script example : {:s}".format(l_example)))
+    user_choice=input("Do you want to generate the file {:s} in the current directory? [yes/No] ".format(l_example))
     if((user_choice=='yes')or(user_choice=='y')):
       fh_output = open(l_example, 'w')
       fh_output.write(ceg_example_list[l_example])
       fh_output.close()
-      print(ceg_instructions.format(l_example, l_example, l_example))
+      six.print_((ceg_instructions.format(l_example, l_example, l_example)))
     else:
-      print("The script example {:s} has not been created.".format(l_example))
+      six.print_(("The script example {:s} has not been created.".format(l_example)))
 
 
diff --git a/bin/cnc25d_example_generator_src.py b/bin/cnc25d_example_generator_src.py
index 4028807..5a11502 100755
--- a/bin/cnc25d_example_generator_src.py
+++ b/bin/cnc25d_example_generator_src.py
@@ -33,6 +33,8 @@
 
 import os, re, sys
 import subprocess
+import six
+from six.moves import input
 
 ##############################################################################
 # Checking FreeCAD installation
@@ -76,11 +78,11 @@
 freecad_verion_major = int(freecad_verion[0])
 freecad_verion_minor = int(freecad_verion[1])
 if((freecad_verion_major==0)and(freecad_verion_minor<13)):
-  print("ERR056: Error, Your FreeCAD version is too old! You have {:d}.{:d} and 0.13 or newer is needed.".format(freecad_verion_major, freecad_verion_minor))
+  six.print_(("ERR056: Error, Your FreeCAD version is too old! You have {:d}.{:d} and 0.13 or newer is needed.".format(freecad_verion_major, freecad_verion_minor)))
   print("Please, install the latest FreeCAD version on your system and re-run this script.")
   sys.exit(1)
 #info
-print("The FreeCAD version {:d}.{:d} is installed on your system.".format(freecad_verion_major, freecad_verion_minor))
+six.print_(("The FreeCAD version {:d}.{:d} is installed on your system.".format(freecad_verion_major, freecad_verion_minor)))
 
 ### check if the FreeCAD Library can be imported
 
@@ -317,8 +319,8 @@
 ceg_example_list_sorted_keys = sorted(ceg_example_list.keys())
 print("\nThis executable helps you to generate the following cnc25d script examples in the current directory:")
 for l_example in ceg_example_list_sorted_keys:
-  print("  +  {:s}".format(l_example))
-user_choice=raw_input("Do you want to generate all these upper files in the current directory? [yes/No] ")
+  six.print_(("  +  {:s}".format(l_example)))
+user_choice=input("Do you want to generate all these upper files in the current directory? [yes/No] ")
 if((user_choice=='yes')or(user_choice=='y')):
   for l_example in ceg_example_list_sorted_keys:
     fh_output = open(l_example, 'w')
@@ -328,14 +330,14 @@
 else:
   print("Choose which cnc25d script example you want to generate in the current directory:")
   for l_example in ceg_example_list_sorted_keys:
-    print("cnc25d script example : {:s}".format(l_example))
-    user_choice=raw_input("Do you want to generate the file {:s} in the current directory? [yes/No] ".format(l_example))
+    six.print_(("cnc25d script example : {:s}".format(l_example)))
+    user_choice=input("Do you want to generate the file {:s} in the current directory? [yes/No] ".format(l_example))
     if((user_choice=='yes')or(user_choice=='y')):
       fh_output = open(l_example, 'w')
       fh_output.write(ceg_example_list[l_example])
       fh_output.close()
-      print(ceg_instructions.format(l_example, l_example, l_example))
+      six.print_((ceg_instructions.format(l_example, l_example, l_example)))
     else:
-      print("The script example {:s} has not been created.".format(l_example))
+      six.print_(("The script example {:s} has not been created.".format(l_example)))
 
 
diff --git a/cnc25d/angles.py b/cnc25d/angles.py
index 43a3f2b..832347b 100644
--- a/cnc25d/angles.py
+++ b/cnc25d/angles.py
@@ -30,6 +30,7 @@
 ################################################################
 
 import math
+import six
 
 ################################################################
 # functions
@@ -53,7 +54,7 @@ def roll_pitch_to_pan_tilt(ai_a1, ai_a2):
       if(abs(sin_A)<1+radian_epsilon):
         sin_A = math.copysign(1, sin_A)
       else:
-        print("ERR053: Internal Error, sin_A {:0.3f}".format(sin_A))
+        six.print_(("ERR053: Internal Error, sin_A {:0.3f}".format(sin_A)))
         sys.exit(2)
     A = math.asin(sin_A)
     b1 = b1_sign*math.pi/2 - A
diff --git a/cnc25d/axle_lid.py b/cnc25d/axle_lid.py
index 7cb8653..d1b2d58 100644
--- a/cnc25d/axle_lid.py
+++ b/cnc25d/axle_lid.py
@@ -29,7 +29,8 @@
 # header for Python / FreeCAD compatibility
 ################################################################
 
-import cnc25d_api
+from . import cnc25d_api
+import six
 cnc25d_api.importing_freecad()
 
 #print("FreeCAD.Version:", FreeCAD.Version())
@@ -52,7 +53,7 @@
 #import svgwrite
 #from dxfwrite import DXFEngine
 # cnc25d
-import gearring
+from . import gearring
 
 ################################################################
 # inheritance from gearring
@@ -225,19 +226,19 @@ def axle_lid_constraint_check(c):
   #  print("ERR141: Error, cnc_router_bit_radius {:0.3f} is bigger than axle_hole_radius {:0.3f}".format(c['cnc_router_bit_radius'], c['axle_hole_radius']))
   #  sys.exit(2)
   if(c['axle_hole_radius']>c['central_radius']-radian_epsilon):
-    print("ERR144: Error, axle_hole_radius {:0.3f} is bigger than central_radius {:0.3f}".format(c['axle_hole_radius'], c['central_radius']))
+    six.print_(("ERR144: Error, axle_hole_radius {:0.3f} is bigger than central_radius {:0.3f}".format(c['axle_hole_radius'], c['central_radius'])))
     sys.exit(2)
   if(c['central_radius']>c['clearance_radius']-radian_epsilon):
-    print("ERR147: Error, central_radius {:0.3f} is bigger than clearance_radius {:0.3f}".format(c['central_radius'], c['clearance_radius']))
+    six.print_(("ERR147: Error, central_radius {:0.3f} is bigger than clearance_radius {:0.3f}".format(c['central_radius'], c['clearance_radius'])))
     sys.exit(2)
   if(c['clearance_radius']>c['holder_radius']-radian_epsilon):
-    print("ERR151: Error, clearance_radius {:0.3f} is bigger than the holder_radius {:0.3f}".format(c['clearance_radius'], c['holder_radius']))
+    six.print_(("ERR151: Error, clearance_radius {:0.3f} is bigger than the holder_radius {:0.3f}".format(c['clearance_radius'], c['holder_radius'])))
     sys.exit(2)
   if(c['annulus_holder_axle_hole_radius']>c['clearance_radius']):
-    print("ERR159: Error, annulus_holder_axle_hole_radius {:0.3f} is bigger than clearance_radius {:0.3f}".format(c['annulus_holder_axle_hole_radius'], c['clearance_radius']))
+    six.print_(("ERR159: Error, annulus_holder_axle_hole_radius {:0.3f} is bigger than clearance_radius {:0.3f}".format(c['annulus_holder_axle_hole_radius'], c['clearance_radius'])))
     sys.exit(2)
   if(c['holder_crenel_number']<4):
-    print("ERR154: Error, holder_crenel_number {:d} is smaller than 4".format(c['holder_crenel_number']))
+    six.print_(("ERR154: Error, holder_crenel_number {:d} is smaller than 4".format(c['holder_crenel_number'])))
     sys.exit(2)
   c['middle_crenel_1'] = 0
   c['middle_crenel_2'] = int(c['holder_crenel_number']/2)
@@ -267,22 +268,22 @@ def axle_lid_constraint_check(c):
     c['leg_hole_distance'] = 2*(2*c['leg_hole_radius'] + c['leg_hole_length'])
   if(c['leg_type'] != 'none'):
     if((c['leg_type'] != 'rear') and (c['leg_type'] != 'side')):
-      print("ERR536: Error, leg_type {:s} set to an unknow value. Possible values: 'none', 'rear' or 'side'".format(c['leg_type']))
+      six.print_(("ERR536: Error, leg_type {:s} set to an unknow value. Possible values: 'none', 'rear' or 'side'".format(c['leg_type'])))
       sys.exit(2)
     if(c['leg_length']<=0):
-      print("ERR539: Error, leg_length {:0.3f} must be strictly positive".format(c['leg_length']))
+      six.print_(("ERR539: Error, leg_length {:0.3f} must be strictly positive".format(c['leg_length'])))
       sys.exit(2)
     if(c['toe_length']<c['leg_hole_radius']):
-      print("ERR543: Error, toe_length {:0.3f} is smaller than leg_hole_radius {:0.3f}".format(c['toe_length'], c['leg_hole_radius']))
+      six.print_(("ERR543: Error, toe_length {:0.3f} is smaller than leg_hole_radius {:0.3f}".format(c['toe_length'], c['leg_hole_radius'])))
       sys.exit(2)
     #if(c['foot_length']<c['leg_hole_radius']):
     #  print("ERR543: Error, foot_length {:0.3f} is smaller than leg_hole_radius {:0.3f}".format(c['foot_length'], c['leg_hole_radius']))
     #  sys.exit(2)
     if(c['leg_hole_distance']<2*c['leg_hole_radius']+c['leg_hole_length']):
-      print("ERR549: Error, leg_hole_distance {:0.3f} is too small compare to leg_hole_radius {:0.3f} and leg_hole_length {:0.3f}".format(c['leg_hole_distance'], c['leg_hole_radius'], c['leg_hole_length']))
+      six.print_(("ERR549: Error, leg_hole_distance {:0.3f} is too small compare to leg_hole_radius {:0.3f} and leg_hole_length {:0.3f}".format(c['leg_hole_distance'], c['leg_hole_radius'], c['leg_hole_length'])))
       sys.exit(2)
     if(c['leg_border_length']<c['leg_hole_radius']):
-      print("ERR552: Error, leg_border_length {:0.3f} is smaller than leg_hole_radius {:0.3f}".format(c['leg_border_length'], c['leg_hole_radius']))
+      six.print_(("ERR552: Error, leg_border_length {:0.3f} is smaller than leg_hole_radius {:0.3f}".format(c['leg_border_length'], c['leg_hole_radius'])))
       sys.exit(2)
   ### axle_B
   c['output_axle_B_internal_radius'] = c['output_axle_B_internal_diameter']/2.0
@@ -464,27 +465,27 @@ def axle_lid_2d_construction(c):
   ### axle_B
   if(c['output_axle_B_place'] != 'none'):
     if((c['output_axle_B_place'] != 'small')and(c['output_axle_B_place'] != 'large')):
-      print("ERR442: Error, output_axle_B_place {:s} is unknown. Possible values: 'none', 'small' or 'large'".format(c['output_axle_B_place']))
+      six.print_(("ERR442: Error, output_axle_B_place {:s} is unknown. Possible values: 'none', 'small' or 'large'".format(c['output_axle_B_place'])))
       sys.exit(2)
     if(c['output_axle_distance']<=0.0):
-      print("ERR445: Error, output_axle_distance {:0.3f} must be positive".format(c['output_axle_distance']))
+      six.print_(("ERR445: Error, output_axle_distance {:0.3f} must be positive".format(c['output_axle_distance'])))
       sys.exit(2)
     if(c['output_axle_B_external_radius'] == 0):
       c['output_axle_B_external_radius'] = 2*c['output_axle_B_internal_radius']
     if(c['output_axle_B_external_radius']<=c['output_axle_B_internal_radius']):
-      print("ERR452: Error, output_axle_B_external_radius {:0.3f} is null or smaller than output_axle_B_internal_radius {:0.3f}".format(c['output_axle_B_external_radius'], c['output_axle_B_internal_radius']))
+      six.print_(("ERR452: Error, output_axle_B_external_radius {:0.3f} is null or smaller than output_axle_B_internal_radius {:0.3f}".format(c['output_axle_B_external_radius'], c['output_axle_B_internal_radius'])))
       sys.exit(2)
     if(c['output_axle_B_crenel_number']>0):
       if(c['output_axle_B_crenel_radius']<radian_epsilon):
-        print("ERR460: Error, output_axle_B_crenel_radius {:0.3f} is too small".format(c['output_axle_B_crenel_radius']))
+        six.print_(("ERR460: Error, output_axle_B_crenel_radius {:0.3f} is too small".format(c['output_axle_B_crenel_radius'])))
         sys.exit(2)
       if(c['output_axle_B_crenel_position_radius'] == 0):
         c['output_axle_B_crenel_position_radius'] = (c['output_axle_B_internal_radius'] + c['output_axle_B_external_radius'])/2.0
       if(c['output_axle_B_crenel_position_radius']<(c['output_axle_B_internal_radius']+c['output_axle_B_crenel_radius'])):
-        print("ERR460: Error, output_axle_B_crenel_position_radius {:0.3f} is too small compare to output_axle_B_internal_radius {:0.3f} and output_axle_B_crenel_radius {:0.3f}".format(c['output_axle_B_crenel_position_radius'], c['output_axle_B_internal_radius'], c['output_axle_B_crenel_radius']))
+        six.print_(("ERR460: Error, output_axle_B_crenel_position_radius {:0.3f} is too small compare to output_axle_B_internal_radius {:0.3f} and output_axle_B_crenel_radius {:0.3f}".format(c['output_axle_B_crenel_position_radius'], c['output_axle_B_internal_radius'], c['output_axle_B_crenel_radius'])))
         sys.exit(2)
       if(c['output_axle_B_crenel_position_radius']>(c['output_axle_B_external_radius']-c['output_axle_B_crenel_radius'])):
-        print("ERR463: Error, output_axle_B_crenel_position_radius {:0.3f} is too big compare to output_axle_B_external_radius {:0.3f} and output_axle_B_crenel_radius {:0.3f}".format(c['output_axle_B_crenel_position_radius'], c['output_axle_B_external_radius'], c['output_axle_B_crenel_radius']))
+        six.print_(("ERR463: Error, output_axle_B_crenel_position_radius {:0.3f} is too big compare to output_axle_B_external_radius {:0.3f} and output_axle_B_crenel_radius {:0.3f}".format(c['output_axle_B_crenel_position_radius'], c['output_axle_B_external_radius'], c['output_axle_B_crenel_radius'])))
         sys.exit(2)
     ## calculation of the angle (DCE)
     ABC = (c['middle_crenel_2'] - c['middle_crenel_1'] - 1) * c['crenel_portion_angle'] / 2.0 - c['holder_crenel_half_angle']
@@ -495,7 +496,7 @@ def axle_lid_2d_construction(c):
     AC1 = math.sqrt(AB**2 + BC**2 - 2*AB*BC*math.cos(ABC1))
     cos_ACB1 = ((AC1**2+BC**2-AB**2)/(2*AC1*BC))
     if((cos_ACB1<radian_epsilon)or(cos_ACB1>1-radian_epsilon)):
-      print("ERR474: Error, cos_ACB1 {:0.3f} is out of the range 0..1".format(cos_ACB1))
+      six.print_(("ERR474: Error, cos_ACB1 {:0.3f} is out of the range 0..1".format(cos_ACB1)))
       sys.exit(2)
     ACB1 = math.acos(cos_ACB1)
     DC = c['output_axle_B_external_radius']
@@ -539,10 +540,10 @@ def axle_lid_2d_construction(c):
     # external_axle_B_half_angle
     external_axle_B_half_angle = math.atan(float(input_axle_B_external_radius)/c['holder_radius'])
     if(first_angle>c['output_axle_B_angle']-external_axle_B_half_angle):
-      print("ERR500: Error, first_angle {:0.3f} too big compare to output_axle_B_angle {:0.3f} and external_axle_B_half_angle {:0.3f}".format(first_angle, c['output_axle_B_angle'], external_axle_B_half_angle))
+      six.print_(("ERR500: Error, first_angle {:0.3f} too big compare to output_axle_B_angle {:0.3f} and external_axle_B_half_angle {:0.3f}".format(first_angle, c['output_axle_B_angle'], external_axle_B_half_angle)))
       sys.exit(2)
     if(last_angle<c['output_axle_B_angle']+external_axle_B_half_angle):
-      print("ERR503: Error, last_angle {:0.3f} too small compare to output_axle_B_angle {:0.3f} and external_axle_B_half_angle {:0.3f}".format(last_angle, c['output_axle_B_angle'], external_axle_B_half_angle))
+      six.print_(("ERR503: Error, last_angle {:0.3f} too small compare to output_axle_B_angle {:0.3f} and external_axle_B_half_angle {:0.3f}".format(last_angle, c['output_axle_B_angle'], external_axle_B_half_angle)))
       sys.exit(2)
     holder_A = []
     # first point
diff --git a/cnc25d/bagel.py b/cnc25d/bagel.py
index 8041971..71558e2 100644
--- a/cnc25d/bagel.py
+++ b/cnc25d/bagel.py
@@ -29,7 +29,8 @@
 # header for Python / FreeCAD compatibility
 ################################################################
 
-import cnc25d_api
+from . import cnc25d_api
+import six
 cnc25d_api.importing_freecad()
 
 #print("FreeCAD.Version:", FreeCAD.Version())
@@ -110,21 +111,21 @@ def bagel_constraint_check(c):
   # bagel_axle_diameter
   c['bagel_axle_radius'] = c['bagel_axle_diameter']/2.0
   if(c['bagel_axle_radius']<radian_epsilon):
-    print("ERR152: Error, bagel_axle_radius {:0.3f} is too small".format(c['bagel_axle_radius']))
+    six.print_(("ERR152: Error, bagel_axle_radius {:0.3f} is too small".format(c['bagel_axle_radius'])))
     sys.exit(2)
   # bagel_axle_internal_diameter
   c['bagel_axle_internal_radius'] = c['bagel_axle_internal_diameter']/2.0
   if(c['bagel_axle_internal_radius']==0):
     c['bagel_axle_internal_radius'] = 2*c['bagel_axle_radius']
   if(c['bagel_axle_internal_radius']<c['bagel_axle_radius']):
-    print("ERR159: Error, bagel_axle_internal_radius {:0.3f} must be bigger than bagel_axle_radius {:0.3f}".format(c['bagel_axle_internal_radius'], c['bagel_axle_radius']))
+    six.print_(("ERR159: Error, bagel_axle_internal_radius {:0.3f} must be bigger than bagel_axle_radius {:0.3f}".format(c['bagel_axle_internal_radius'], c['bagel_axle_radius'])))
     sys.exit(2)
   # bagel_axle_external_diameter
   c['bagel_axle_external_radius'] = c['bagel_axle_external_diameter']/2.0
   if(c['bagel_axle_external_radius']==0):
     c['bagel_axle_external_radius'] = 2*c['bagel_axle_internal_radius']
   if(c['bagel_axle_external_radius']<c['bagel_axle_internal_radius']+radian_epsilon):
-    print("ERR166: Error, bagel_axle_external_radius {:0.3f} must be bigger than bagel_axle_internal_radius {:0.3f}".format(c['bagel_axle_external_radius'], c['bagel_axle_internal_radius']))
+    six.print_(("ERR166: Error, bagel_axle_external_radius {:0.3f} must be bigger than bagel_axle_internal_radius {:0.3f}".format(c['bagel_axle_external_radius'], c['bagel_axle_internal_radius'])))
     sys.exit(2)
   # axle_hole_nb
   c['axle_hole_radius'] = 0.0
@@ -133,34 +134,34 @@ def bagel_constraint_check(c):
     # axle_hole_diameter
     c['axle_hole_radius'] = c['axle_hole_diameter']/2.0
     if(c['axle_hole_radius']<radian_epsilon):
-      print("ERR173: Error, axle_hole_radius {:0.3f} must be strictly positive".format(c['axle_hole_radius']))
+      six.print_(("ERR173: Error, axle_hole_radius {:0.3f} must be strictly positive".format(c['axle_hole_radius'])))
       sys.exit(2)
     # axle_hole_position_diameter
     c['axle_hole_position_radius'] = c['axle_hole_position_diameter']/2.0
     if(c['axle_hole_position_radius']==0.0):
       c['axle_hole_position_radius'] = (c['bagel_axle_internal_radius']+c['bagel_axle_external_radius'])/2.0
     if(c['axle_hole_position_radius'] < c['bagel_axle_internal_radius']+c['axle_hole_radius']+radian_epsilon):
-      print("ERR180: Error: axle_hole_position_radius {:0.3f} is too small compare to bagel_axle_internal_radius {:0.3f} and axle_hole_radius {:0.3f}".format(c['axle_hole_position_radius'], c['bagel_axle_internal_radius'], c['axle_hole_radius']))
+      six.print_(("ERR180: Error: axle_hole_position_radius {:0.3f} is too small compare to bagel_axle_internal_radius {:0.3f} and axle_hole_radius {:0.3f}".format(c['axle_hole_position_radius'], c['bagel_axle_internal_radius'], c['axle_hole_radius'])))
       sys.exit(2)
     if(c['axle_hole_position_radius'] > c['bagel_axle_external_radius']-c['axle_hole_radius']-radian_epsilon):
-      print("ERR183: Error: axle_hole_position_radius {:0.3f} is too big compare to bagel_axle_external_radius {:0.3f} and axle_hole_radius {:0.3f}".format(c['axle_hole_position_radius'], c['bagel_axle_external_radius'], c['axle_hole_radius']))
+      six.print_(("ERR183: Error: axle_hole_position_radius {:0.3f} is too big compare to bagel_axle_external_radius {:0.3f} and axle_hole_radius {:0.3f}".format(c['axle_hole_position_radius'], c['bagel_axle_external_radius'], c['axle_hole_radius'])))
       sys.exit(2)
     # axle_hole_angle
   # external_bagel_thickness
   if(c['external_bagel_thickness']<radian_epsilon):
-    print("ERR188: Error, external_bagel_thickness {:0.3f} is too small".format(c['external_bagel_thickness']))
+    six.print_(("ERR188: Error, external_bagel_thickness {:0.3f} is too small".format(c['external_bagel_thickness'])))
     sys.exit(2)
   # middle_bagel_thickness
   if(c['middle_bagel_thickness']<radian_epsilon):
-    print("ERR192: Error, middle_bagel_thickness {:0.3f} is too small".format(c['middle_bagel_thickness']))
+    six.print_(("ERR192: Error, middle_bagel_thickness {:0.3f} is too small".format(c['middle_bagel_thickness'])))
     sys.exit(2)
   # internal_bagel_thickness
   if(c['internal_bagel_thickness']<radian_epsilon):
-    print("ERR196: Error, internal_bagel_thickness {:0.3f} is too small".format(c['internal_bagel_thickness']))
+    six.print_(("ERR196: Error, internal_bagel_thickness {:0.3f} is too small".format(c['internal_bagel_thickness'])))
     sys.exit(2)
   # bagel_extra_cut_thickness
   if(abs(c['bagel_extra_cut_thickness'])>c['bagel_axle_radius']/2.0):
-    print("ERR212: Error, bagel_extra_cut_thickness {:0.3f} is too big compare to bagel_axle_radius {:0.3f}".format(c['bagel_extra_cut_thickness'], c['bagel_axle_radius']))
+    six.print_(("ERR212: Error, bagel_extra_cut_thickness {:0.3f} is too big compare to bagel_axle_radius {:0.3f}".format(c['bagel_extra_cut_thickness'], c['bagel_axle_radius'])))
     sys.exit(2)
   return(c)
 
diff --git a/cnc25d/bare_design.py b/cnc25d/bare_design.py
index 2870a16..1d888db 100644
--- a/cnc25d/bare_design.py
+++ b/cnc25d/bare_design.py
@@ -34,9 +34,10 @@
 from datetime import datetime
 import os
 #
-import outline_backends
-import design_help
-import design_output
+from . import outline_backends
+from . import design_help
+from . import design_output
+import six
 
 ################################################################
 # bare_design class
@@ -188,7 +189,7 @@ def apply_constraint(self, constraint):
     new_contraint_value = 0
     for k in constraint.keys():
       if(not k in rc.keys()):
-        print("ERR177: Error, contraint {:k} is not part of the design".format(k))
+        six.print_(("ERR177: Error, contraint {:k} is not part of the design".format(k)))
         sys.exit(2)
       if(c[k] != constraint[k]):
         new_contraint_value += 1
@@ -215,7 +216,7 @@ def apply_constraint(self, constraint):
   def apply_external_constraint(self, constraint):
     """ set the dictionary constraint to the design without generating error on unknow constraint
     """
-    key_list = self.reference_constraint.viewkeys() & constraint.viewkeys()
+    key_list = self.reference_constraint & constraint
     c =  dict([ (k, constraint[k]) for k in key_list ])
     #print("dbg207: {:s} apply_external_constraint: constraint_nb: {:d}".format(self.design_name, len(c)))
     #print("dbg170: c:", c)
@@ -307,7 +308,7 @@ def get_A_figure(self, figure_id=""):
       figure_id = self.A_figures.keys()[0]
     #print("dbg194: figure_id:", figure_id)
     if(not figure_id in self.A_figures.keys()):
-      print("ERR156: Error, figure_id {:s} is not in the figure list [{:s}]".format(figure_id, ' '.join(self.A_figures.keys())))
+      six.print_(("ERR156: Error, figure_id {:s} is not in the figure list [{:s}]".format(figure_id, ' '.join(self.A_figures.keys()))))
       sys.exit(2)
     r_fig = self.A_figures[figure_id]
     return(r_fig)
@@ -333,20 +334,20 @@ def get_display_2d_figure_list(self):
         r_list = figs
       for f in r_list:
         if(not f in self.A_figures.keys()):
-          print("ERR304: Error, f {:s} is not an existing 2d-figures {:s}".format(f, ' '.join(self.A_figures.keys())))
+          six.print_(("ERR304: Error, f {:s} is not an existing 2d-figures {:s}".format(f, ' '.join(self.A_figures.keys()))))
           sys.exit(2)
     return(r_list)
 
   def outline_display(self):
     """ display in one or several Tk windows the design outline figures
     """
-    print("{:s}".format(self.get_info()))
+    six.print_(("{:s}".format(self.get_info())))
     fig_ids = self.get_display_2d_figure_list()
     for f in fig_ids:
       fig = self.A_figures[f]
       d_info = "display_{:s}".format(f)
       #print("dbg218: fig:", fig)
-      print("{:s}".format(d_info))
+      six.print_(("{:s}".format(d_info)))
       outline_backends.figure_simple_display(design_output.cnc_cut_figure(fig, d_info), design_output.ideal_figure(fig, d_info), d_info)
 
   def apply_3d_constructor(self):
@@ -395,7 +396,7 @@ def get_fc_obj_3dconf(self, assembly_name=""):
     if(assembly_name==""):
       assembly_name = self.assembly_configurations.keys()[0] # set the first 3d-assembly-configuration per default
     if(not assembly_name in self.assembly_configurations.keys()):
-      print("ERR187: Error, the assembly_name {:s} is not in the possible 3d-assembly-configuration".format(assembly_name))
+      six.print_(("ERR187: Error, the assembly_name {:s} is not in the possible 3d-assembly-configuration".format(assembly_name)))
       sys.exit(2)
     #print("dbg245: self.assembly_configurations:", self.assembly_configurations)
     #print("dbg246: assembly_name:", assembly_name)
@@ -411,7 +412,7 @@ def get_fc_obj_function(self, function_id=""):
     if(function_id==""):
       function_id = self.freecad_function_pts.keys()[0] # set the first 3d-freecad_function_pts per default
     if(not function_id in self.freecad_function_pts.keys()):
-      print("ERR370: Error, the function_id {:s} is not in the possible 3d-freecad_function_pts".format(function_id))
+      six.print_(("ERR370: Error, the function_id {:s} is not in the possible 3d-freecad_function_pts".format(function_id)))
       sys.exit(2)
     r_fc_obj = self.freecad_function_pts[function_id](self.constraint) # execute the function that generate a freecad object
     return(r_fc_obj)
@@ -424,7 +425,7 @@ def write_info_txt(self, output_file_basename):
     design_help.mkdir_p(l_output_dir)
     #
     info_txt_filename = "{:s}_info.txt".format(output_file_basename)
-    print("Generate the text info file {:s}".format(info_txt_filename))
+    six.print_(("Generate the text info file {:s}".format(info_txt_filename)))
     ofh = open(info_txt_filename, 'w')
     ofh.write("{:s} generated by Cnc25D on {:s}\n\n".format(info_txt_filename, datetime.now().isoformat()))
     ofh.write(self.get_info())
@@ -443,7 +444,7 @@ def get_write_2d_figure_list(self):
         r_list = figs
       for f in r_list:
         if(not f in self.A_figures.keys()):
-          print("ERR291: Error, f {:s} is not an existing 2d-figures {:s}".format(f, ' '.join(self.A_figures.keys())))
+          six.print_(("ERR291: Error, f {:s} is not an existing 2d-figures {:s}".format(f, ' '.join(self.A_figures.keys()))))
           sys.exit(2)
     return(r_list)
 
@@ -478,7 +479,7 @@ def get_write_3d_figure_list(self):
         r_list = figs
       for f in r_list:
         if(not f in self.A_figures.keys()):
-          print("ERR380: Error, f {:s} is not an existing 2d-figures {:s}".format(f, ' '.join(self.A_figures.keys())))
+          six.print_(("ERR380: Error, f {:s} is not an existing 2d-figures {:s}".format(f, ' '.join(self.A_figures.keys()))))
           sys.exit(2)
     return(r_list)
 
@@ -504,7 +505,7 @@ def get_write_3d_conf_list(self):
         r_list = confs
       for f in r_list:
         if(not f in self.assembly_configurations.keys()):
-          print("ERR406: Error, f {:s} is not an existing 3d-assembly-configurations {:s}".format(f, ' '.join(self.assembly_configurations.keys())))
+          six.print_(("ERR406: Error, f {:s} is not an existing 3d-assembly-configurations {:s}".format(f, ' '.join(self.assembly_configurations.keys()))))
           sys.exit(2)
     return(r_list)
 
@@ -514,7 +515,7 @@ def write_assembly_brep(self, output_file_basename, ai_brep=True, ai_stl=False):
     """
     confs = self.get_write_3d_conf_list()
     for a in confs:
-      print("write_assembly_brep: {:s}".format(a))
+      six.print_(("write_assembly_brep: {:s}".format(a)))
       # (ai_3d_conf, ai_output_filename, ai_brep=True, ai_stl=False, ai_slice_xyz=[])
       design_output.generate_3d_assembly_output_file(self.complete_assembly_conf(self.assembly_configurations[a]), "{:s}_{:s}".format(output_file_basename, a), ai_brep=ai_brep, ai_stl=ai_stl, ai_slice_xyz=self.slice3d_configurations[a]) 
 
@@ -532,7 +533,7 @@ def get_write_3d_freecad_list(self):
         r_list = l
       for f in r_list:
         if(not f in self.freecad_function_pts.keys()):
-          print("ERR460: Error, f {:s} is not an existing 3d-freecad_function_pts {:s}".format(f, ' '.join(self.freecad_function_pts.keys())))
+          six.print_(("ERR460: Error, f {:s} is not an existing 3d-freecad_function_pts {:s}".format(f, ' '.join(self.freecad_function_pts.keys()))))
           sys.exit(2)
     return(r_list)
 
@@ -549,14 +550,14 @@ def run_simulation(self, sim_id=''):
     """ run the simulation sim_id
     """
     if(len(self.simulation_2d_pts)==0):
-      print("ERR268: Error, no simulation function is provided. Can't run 2d-simulate {:s}".format(sim_id))
+      six.print_(("ERR268: Error, no simulation function is provided. Can't run 2d-simulate {:s}".format(sim_id)))
       sys.exit(2)
     if(sim_id==''):
       sim_id = self.simulation_2d_pts.keys()[0]
     if(not sim_id in self.simulation_2d_pts.keys()):
-      print("ERR382: Error, the simulation id {:s} does not exist in the list {:s}".format(sim_id, ' '.join(self.simulation_2d_pts.keys())))
+      six.print_(("ERR382: Error, the simulation id {:s} does not exist in the list {:s}".format(sim_id, ' '.join(self.simulation_2d_pts.keys()))))
       sys.exit(2)
-    print("SIMULATION: {:s} runs simulation: {:s}".format(self.design_name, sim_id))
+    six.print_(("SIMULATION: {:s} runs simulation: {:s}".format(self.design_name, sim_id)))
     #self.simulation_2d_pts[sim_id](self.constraint) # writing correct but too compact for my eyes ;)
     f_simulation = self.simulation_2d_pts[sim_id]
     f_simulation(self.constraint)
@@ -568,70 +569,70 @@ def view_design_configuration(self):
     # 2D-figures
     (figs, height) = self.apply_2d_constructor()
     fig_ids = figs.keys()
-    print("{:s} 2D-figures:".format(self.design_name))
+    six.print_(("{:s} 2D-figures:".format(self.design_name)))
     if(len(fig_ids)==0):
       print("No 2D-figures")
     else:
       for i in range(len(fig_ids)):
-        print("{:02d} : {:s}".format(i, fig_ids[i]))
+        six.print_(("{:02d} : {:s}".format(i, fig_ids[i])))
     # 2D-simulations
     sim_ids = self.simulation_2d_pts.keys()
-    print("{:s} 2D-simulations:".format(self.design_name))
+    six.print_(("{:s} 2D-simulations:".format(self.design_name)))
     if(len(sim_ids)==0):
       print("No 2D-simulation")
     else:
       for i in range(len(sim_ids)):
-        print("{:02d} : {:s}".format(i, sim_ids[i]))
+        six.print_(("{:02d} : {:s}".format(i, sim_ids[i])))
     # assembly_3d_configurations
     if(self.f_3d_constructor==None):
       print("No assembly_3dconf")
     else:
       self.apply_3d_constructor()
       assembly_ids = self.assembly_configurations.keys()
-      print("{:s} assembly_3dconf:".format(self.design_name))
+      six.print_(("{:s} assembly_3dconf:".format(self.design_name)))
       for i in range(len(assembly_ids)):
-        print("{:02d} : {:s}".format(i, assembly_ids[i]))
+        six.print_(("{:02d} : {:s}".format(i, assembly_ids[i])))
     # assembly_3d_freecad_configurations
     if(self.f_3d_freecad_constructor==None):
       print("No freecad_3dconf")
     else:
       self.apply_3d_freecad_constructor()
       fc_ids = self.freecad_function_pts.keys()
-      print("{:s} freecad_function_pts:".format(self.design_name))
+      six.print_(("{:s} freecad_function_pts:".format(self.design_name)))
       for i in range(len(fc_ids)):
-        print("{:02d} : {:s}".format(i, fc_ids[i]))
+        six.print_(("{:02d} : {:s}".format(i, fc_ids[i])))
     # display_2d_figure_list
-    print("{:s} display_2d_figure_list:".format(self.design_name))
+    six.print_(("{:s} display_2d_figure_list:".format(self.design_name)))
     figs = self.get_display_2d_figure_list()
     for i in range(len(figs)):
-      print("{:02d} : {:s}".format(i, figs[i]))
+      six.print_(("{:02d} : {:s}".format(i, figs[i])))
     # default_simulation
-    print("{:s} default_simulation: {:s}".format(self.design_name, self.default_simulation))
+    six.print_(("{:s} default_simulation: {:s}".format(self.design_name, self.default_simulation)))
     # file_2d_figure_file_list
-    print("{:s} svg-dxf-file_2d_figure_list:".format(self.design_name))
+    six.print_(("{:s} svg-dxf-file_2d_figure_list:".format(self.design_name)))
     figs = self.get_write_2d_figure_list()
     for i in range(len(figs)):
-      print("{:02d} : {:s}".format(i, figs[i]))
+      six.print_(("{:02d} : {:s}".format(i, figs[i])))
     # file_3d_figure_file_list
-    print("{:s} brep-file_3d_figure_list:".format(self.design_name))
+    six.print_(("{:s} brep-file_3d_figure_list:".format(self.design_name)))
     figs = self.get_write_3d_figure_list()
     for i in range(len(figs)):
-      print("{:02d} : {:s}".format(i, figs[i]))
+      six.print_(("{:02d} : {:s}".format(i, figs[i])))
     # file_3d_conf_file_list
-    print("{:s} brep-file_3d_conf_list:".format(self.design_name))
+    six.print_(("{:s} brep-file_3d_conf_list:".format(self.design_name)))
     confs = self.get_write_3d_conf_list()
     for i in range(len(confs)):
-      print("{:02d} : {:s}".format(i, confs[i]))
+      six.print_(("{:02d} : {:s}".format(i, confs[i])))
     # file_3d_freecad_file_list
-    print("{:s} brep-file_3d_freecad_list:".format(self.design_name))
+    six.print_(("{:s} brep-file_3d_freecad_list:".format(self.design_name)))
     fc_l = self.get_write_3d_freecad_list()
     for i in range(len(fc_l)):
-      print("{:02d} : {:s}".format(i, fc_l[i]))
+      six.print_(("{:02d} : {:s}".format(i, fc_l[i])))
     # self-test-list
     test_ids = []
     for i in range(len(self.self_tests)):
       test_ids.append(self.self_tests[i][0])
-    print("design_self-test_list: {:s}".format(', '.join(test_ids)))
+    six.print_(("design_self-test_list: {:s}".format(', '.join(test_ids))))
     # return (not yet used)
     return(fig_ids)
 
@@ -717,7 +718,7 @@ def apply_cli_with_output_options(self, cli_str=""):
     r_cli = 1
     if(oo_args.sw_return_type!=''):
       if(self.f_return_type==None):
-        print("ERR277: Error, no return_type function is provided. Can't apply return_type {:s}".format(oo_args.sw_return_type))
+        six.print_(("ERR277: Error, no return_type function is provided. Can't apply return_type {:s}".format(oo_args.sw_return_type)))
         sys.exit(2)
       r_cli = self.f_return_type(oo_args.sw_return_type, self.constraint)
     return(r_cli)
@@ -737,20 +738,20 @@ def run_self_test(self, test_id=''):
       test_list.extend(test_ids)
     else:
       if(not test_id in test_ids):
-        print("ERR541: Error, test_id {:s} is not in the test-list {:s}".format(test_id, ', '.join(test_ids)))
+        six.print_(("ERR541: Error, test_id {:s} is not in the test-list {:s}".format(test_id, ', '.join(test_ids))))
         sys.exit(2)
       test_list.append(test_id)
-    print("\nInfo: test_list: {:s}\n".format(', '.join(test_list)))
+    six.print_(("\nInfo: test_list: {:s}\n".format(', '.join(test_list))))
     for i in range(len(test_list)):
       tn=-1
       for j in range(len(test_ids)):
         if(test_ids[j]==test_list[i]):
           tn=j
       if(tn==-1):
-        print("ERR562: the test {:s} has not been found".format(test_list[i]))
+        six.print_(("ERR562: the test {:s} has not been found".format(test_list[i])))
         sys.exit(2)
       else:
-        print("{:2d} test case: '{:s}'\nwith switch: {:s}".format(tn+1, self.self_tests[tn][0], self.self_tests[tn][1]))
+        six.print_(("{:2d} test case: '{:s}'\nwith switch: {:s}".format(tn+1, self.self_tests[tn][0], self.self_tests[tn][1])))
         self.apply_constraint_default_value()
         self.apply_cli_with_output_options(self.self_tests[tn][1])
     return(1)
@@ -763,10 +764,10 @@ def dump_constraint_file(self, snippet_filename):
       sys.exit(2)
     constraint_dict = vars(self.parser.parse_args([]))
     py_txt = ""
-    for (k,v) in constraint_dict.iteritems():
+    for (k,v) in six.iteritems(constraint_dict):
       #py_txt += "\t{:30s}\t= {:s}\n".format(k, str(v))
       py_txt += "c[\'{:s}\']\t= {:s}\n".format(k, str(v))
-    print("Generate the python snippet file {:s}".format(snippet_filename))
+    six.print_(("Generate the python snippet file {:s}".format(snippet_filename)))
     ofh = open(snippet_filename, 'w')
     ofh.write("# {:s} generated by cnc25d.bare_design for the design example {:s}\n\n".format(snippet_filename, self.design_name))
     ofh.write(py_txt)
@@ -807,9 +808,9 @@ def cli(self, cli_str=""):
 
 # import for testing bare_design
 #import cnc25d_api # cannot import cnc25d_api because cnc25d_api import bare_design
-import importing_freecad
+from . import importing_freecad
 importing_freecad.importing_freecad()
-import cnc_outline
+from . import cnc_outline
 #print("FreeCAD.Version:", FreeCAD.Version())
 #FreeCAD.Console.PrintMessage("Hello from PrintMessage!\n") # avoid using this method because it is not printed in the FreeCAD GUI
 #
@@ -847,19 +848,19 @@ def cube_constraint_check(c):
       c['height'] = c['length']
     # check the constraint
     if(c['width']<c['length']*0.1):
-      print("ERR129: Error, width {:0.3f} is too small compare to length {:0.3f}".format(c['width'], c['length']))
+      six.print_(("ERR129: Error, width {:0.3f} is too small compare to length {:0.3f}".format(c['width'], c['length'])))
       sys.exit(2)
     if(c['width']>c['length']*10.0):
-      print("ERR132: Error, width {:0.3f} is too big compare to length {:0.3f}".format(c['width'], c['length']))
+      six.print_(("ERR132: Error, width {:0.3f} is too big compare to length {:0.3f}".format(c['width'], c['length'])))
       sys.exit(2)
     if(c['height']<c['length']*0.1):
-      print("ERR135: Error, height {:0.3f} is too small compare to length {:0.3f}".format(c['height'], c['length']))
+      six.print_(("ERR135: Error, height {:0.3f} is too small compare to length {:0.3f}".format(c['height'], c['length'])))
       sys.exit(2)
     if(c['height']>c['length']*10.0):
-      print("ERR138: Error, height {:0.3f} is too big compare to length {:0.3f}".format(c['height'], c['length']))
+      six.print_(("ERR138: Error, height {:0.3f} is too big compare to length {:0.3f}".format(c['height'], c['length'])))
       sys.exit(2)
     if(c['smooth_radius']>0.4*min(c['length'], c['width'])):
-      print("ERR182: Error, smooth_radius {:0.3f} is too big compare to length {:0.3f} or width {:0.3f}".format(c['smooth_radius'], c['length'], c['width']))
+      six.print_(("ERR182: Error, smooth_radius {:0.3f} is too big compare to length {:0.3f} or width {:0.3f}".format(c['smooth_radius'], c['length'], c['width'])))
       sys.exit(2)
     return(c)
 
diff --git a/cnc25d/bell.py b/cnc25d/bell.py
index 9347d53..1907e08 100644
--- a/cnc25d/bell.py
+++ b/cnc25d/bell.py
@@ -29,7 +29,8 @@
 # header for Python / FreeCAD compatibility
 ################################################################
 
-import cnc25d_api
+from . import cnc25d_api
+import six
 cnc25d_api.importing_freecad()
 
 #print("FreeCAD.Version:", FreeCAD.Version())
@@ -52,7 +53,7 @@
 #import svgwrite
 #from dxfwrite import DXFEngine
 # cnc25d
-import bell_outline
+from . import bell_outline
 
 
 ################################################################
@@ -222,7 +223,7 @@ def bell_constraint_check(c):
   radian_epsilon = math.pi/1000
   # axle_internal_diameter
   if(c['axle_internal_diameter']<radian_epsilon):
-    print("ERR333: Error, axle_internal_diameter {:0.3f} is too small".format(c['axle_internal_diameter']))
+    six.print_(("ERR333: Error, axle_internal_diameter {:0.3f} is too small".format(c['axle_internal_diameter'])))
     sys.exit(2)
   c['axle_internal_radius'] = c['axle_internal_diameter']/2.0
   # axle_external_diameter
@@ -230,37 +231,37 @@ def bell_constraint_check(c):
   if(c['axle_external_radius']==0):
     c['axle_external_radius'] = 2*c['axle_internal_radius']
   if(c['axle_external_radius']<c['axle_internal_radius']+radian_epsilon):
-    print("ERR340: Error, axle_external_radius {:0.3f} must be bigger than axle_internal_radius {:0.3f}".format(c['axle_external_radius'], c['axle_internal_radius']))
+    six.print_(("ERR340: Error, axle_external_radius {:0.3f} must be bigger than axle_internal_radius {:0.3f}".format(c['axle_external_radius'], c['axle_internal_radius'])))
     sys.exit(2)
   # leg_length
   if(c['leg_length']<c['axle_internal_radius']):
-    print("ERR346: Error, leg_length {:0.3f} must be bigger than axle_internal_radius {:0.3f}".format(c['leg_length'], c['axle_internal_radius']))
+    six.print_(("ERR346: Error, leg_length {:0.3f} must be bigger than axle_internal_radius {:0.3f}".format(c['leg_length'], c['axle_internal_radius'])))
     sys.exit(2)
   # bell_face_height
   if(c['bell_face_height']<radian_epsilon):
-    print("ERR350: Error, bell_face_height {:0.3f} must be strictly positive".format(c['bell_face_height']))
+    six.print_(("ERR350: Error, bell_face_height {:0.3f} must be strictly positive".format(c['bell_face_height'])))
     sys.exit(2)
   # bell_face_width
   if(c['bell_face_width']<2*c['axle_external_radius']):
-    print("ERR354: Error, bell_face_width {:0.3f} is too small compare too axle_external_radius {:0.3f}".format(c['bell_face_width'], c['axle_external_radius']))
+    six.print_(("ERR354: Error, bell_face_width {:0.3f} is too small compare too axle_external_radius {:0.3f}".format(c['bell_face_width'], c['axle_external_radius'])))
     sys.exit(2)
   # base_diameter
   c['base_radius'] = c['base_diameter']/2.0
   if(c['base_radius']<c['bell_face_width']/10.0*math.sqrt(5**2+1.5**2)):
-    print("ERR357: Error, base_radius {:0.3f} is too small compare to bell_face_width {:0.3f}".format(c['base_radius'], c['bell_face_width']))
+    six.print_(("ERR357: Error, base_radius {:0.3f} is too small compare to bell_face_width {:0.3f}".format(c['base_radius'], c['bell_face_width'])))
     sys.exit(2)
   # face_thickness
   if(c['face_thickness']<radian_epsilon):
-    print("ERR358: Error, face_thickness {:0.3f} must be strictly positive".format(c['face_thickness']))
+    six.print_(("ERR358: Error, face_thickness {:0.3f} must be strictly positive".format(c['face_thickness'])))
     sys.exit(2)
   if(c['face_thickness']>c['bell_face_width']/5.0):
-    print("ERR361: Error, face_thickness {:0.3f} is too big compare to bell_face_width {:0.3f}".format(c['face_thickness'], c['bell_face_width']))
+    six.print_(("ERR361: Error, face_thickness {:0.3f} is too big compare to bell_face_width {:0.3f}".format(c['face_thickness'], c['bell_face_width'])))
     sys.exit(2)
   # side_thickness
   if(c['side_thickness']==0.0):
     c['side_thickness'] = c['face_thickness']
   if(c['side_thickness']>c['bell_face_width']/5.0):
-    print("ERR367: Error, side_thickness {:0.3f} is too big compare to bell_face_width {:0.3f}".format(c['side_thickness'], c['bell_face_width']))
+    six.print_(("ERR367: Error, side_thickness {:0.3f} is too big compare to bell_face_width {:0.3f}".format(c['side_thickness'], c['bell_face_width'])))
     sys.exit(2)
   # base_thickness
   if(c['base_thickness']==0.0):
@@ -272,198 +273,198 @@ def bell_constraint_check(c):
     # axle_hole_diameter
     c['axle_hole_radius'] = c['axle_hole_diameter']/2.0
     if(c['axle_hole_radius']<radian_epsilon):
-      print("ERR370: Error, axle_hole_radius {:0.3f} must be strictly positive".format(c['axle_hole_radius']))
+      six.print_(("ERR370: Error, axle_hole_radius {:0.3f} must be strictly positive".format(c['axle_hole_radius'])))
       sys.exit(2)
     # axle_hole_position_diameter
     c['axle_hole_position_radius'] = c['axle_hole_position_diameter']/2.0
     if(c['axle_hole_position_radius']==0.0):
       c['axle_hole_position_radius'] = (c['axle_internal_radius']+c['axle_external_radius'])/2.0
     if(c['axle_hole_position_radius'] < c['axle_internal_radius']+c['axle_hole_radius']+radian_epsilon):
-      print("ERR378: Error: axle_hole_position_radius {:0.3f} is too small compare to axle_internal_radius {:0.3f} and axle_hole_radius {:0.3f}".format(c['axle_hole_position_radius'], c['axle_internal_radius'], c['axle_hole_radius']))
+      six.print_(("ERR378: Error: axle_hole_position_radius {:0.3f} is too small compare to axle_internal_radius {:0.3f} and axle_hole_radius {:0.3f}".format(c['axle_hole_position_radius'], c['axle_internal_radius'], c['axle_hole_radius'])))
       sys.exit(2)
     if(c['axle_hole_position_radius'] > c['axle_external_radius']-c['axle_hole_radius']-radian_epsilon):
-      print("ERR381: Error: axle_hole_position_radius {:0.3f} is too big compare to axle_external_radius {:0.3f} and axle_hole_radius {:0.3f}".format(c['axle_hole_position_radius'], c['axle_external_radius'], c['axle_hole_radius']))
+      six.print_(("ERR381: Error: axle_hole_position_radius {:0.3f} is too big compare to axle_external_radius {:0.3f} and axle_hole_radius {:0.3f}".format(c['axle_hole_position_radius'], c['axle_external_radius'], c['axle_hole_radius'])))
       sys.exit(2)
     # axle_hole_angle
   # leg_spare_width
   if(c['leg_spare_width'] > (c['bell_face_width']-2*c['axle_external_radius'])/2.0+radian_epsilon):
-    print("ERR385: Error, leg_spare_width {:0.3f} is too big compare to bell_face_width {:0.3f} and axle_external_radius {:0.3f}".format(c['leg_spare_width'], c['bell_face_width'], c['axle_external_radius']))
+    six.print_(("ERR385: Error, leg_spare_width {:0.3f} is too big compare to bell_face_width {:0.3f} and axle_external_radius {:0.3f}".format(c['leg_spare_width'], c['bell_face_width'], c['axle_external_radius'])))
     sys.exit(2)
   # leg_smoothing_radius
   if(c['leg_smoothing_radius']<c['bell_cnc_router_bit_radius']):
-    print("ERR389: Error, leg_smoothing_radius {:0.3f} must be bigger than bell_cnc_router_bit_radius {:0.3f}".format(c['leg_smoothing_radius'], c['bell_cnc_router_bit_radius']))
+    six.print_(("ERR389: Error, leg_smoothing_radius {:0.3f} must be bigger than bell_cnc_router_bit_radius {:0.3f}".format(c['leg_smoothing_radius'], c['bell_cnc_router_bit_radius'])))
     sys.exit(2)
   if(c['leg_smoothing_radius']<c['leg_spare_width']):
-    print("ERR403: Error, leg_smoothing_radius {:0.3f} must be bigger than leg_spare_width {:0.3f}".format(c['leg_smoothing_radius'], c['leg_spare_width']))
+    six.print_(("ERR403: Error, leg_smoothing_radius {:0.3f} must be bigger than leg_spare_width {:0.3f}".format(c['leg_smoothing_radius'], c['leg_spare_width'])))
     sys.exit(2)
   if(c['leg_smoothing_radius']>c['leg_length']):
-    print("ERR392: Error, leg_smoothing_radius {:0.3f} must be bigger than leg_length {:0.3f}".format(c['leg_smoothing_radius'], c['leg_length']))
+    six.print_(("ERR392: Error, leg_smoothing_radius {:0.3f} must be bigger than leg_length {:0.3f}".format(c['leg_smoothing_radius'], c['leg_length'])))
     sys.exit(2)
   # motor_hole_diameter
   c['motor_hole_radius'] = c['motor_hole_diameter']/2.0
   if(c['motor_hole_radius']>0.0):
     # motor_hole_x_distance
     if(c['motor_hole_x_distance']<2*c['motor_hole_radius']+radian_epsilon):
-      print("ERR399: Error, motor_hole_x_distance {:0.3f} is too small compare to motor_hole_radius {:0.3f}".format(c['motor_hole_x_distance'], c['motor_hole_radius']))
+      six.print_(("ERR399: Error, motor_hole_x_distance {:0.3f} is too small compare to motor_hole_radius {:0.3f}".format(c['motor_hole_x_distance'], c['motor_hole_radius'])))
       sys.exit(2)
     if(c['motor_hole_x_distance']>c['bell_face_width']-2*c['side_thickness']-2*c['motor_hole_radius']-radian_epsilon):
-      print("ERR402: Error, motor_hole_x_distance {:0.3f} is too big compare to bell_face_width {:0.3f}, side_thickness {:0.3f} and motor_hole_radius {:0.3f}".format(c['motor_hole_x_distance'], c['bell_face_width'], c['side_thickness'], c['motor_hole_radius']))
+      six.print_(("ERR402: Error, motor_hole_x_distance {:0.3f} is too big compare to bell_face_width {:0.3f}, side_thickness {:0.3f} and motor_hole_radius {:0.3f}".format(c['motor_hole_x_distance'], c['bell_face_width'], c['side_thickness'], c['motor_hole_radius'])))
       sys.exit(2)
     # motor_hole_z_distance
     if(c['motor_hole_z_distance']<2*c['motor_hole_radius']+radian_epsilon):
-      print("ERR406: Error, motor_hole_z_distance {:0.3f} is too small compare to motor_hole_radius {:0.3f}".format(c['motor_hole_z_distance'], c['motor_hole_radius']))
+      six.print_(("ERR406: Error, motor_hole_z_distance {:0.3f} is too small compare to motor_hole_radius {:0.3f}".format(c['motor_hole_z_distance'], c['motor_hole_radius'])))
       sys.exit(2)
     if(c['motor_hole_z_distance']>c['bell_face_height']+c['leg_length']-2*c['motor_hole_radius']-radian_epsilon):
-      print("ERR409: Error, motor_hole_z_distance {:0.3f} is too big compare to bell_face_height {:0.3f}, leg_length {:0.3f} and motor_hole_radius {:0.3f}".format(c['motor_hole_z_distance'], c['bell_face_height'], c['leg_length'], c['motor_hole_radius']))
+      six.print_(("ERR409: Error, motor_hole_z_distance {:0.3f} is too big compare to bell_face_height {:0.3f}, leg_length {:0.3f} and motor_hole_radius {:0.3f}".format(c['motor_hole_z_distance'], c['bell_face_height'], c['leg_length'], c['motor_hole_radius'])))
       sys.exit(2)
     # motor_hole_z_position
     if(c['motor_hole_z_position']<c['motor_hole_radius']+radian_epsilon):
-      print("ERR413: Error, motor_hole_z_position {:0.3f} is too small compare to motor_hole_radius {:0.3f}".format(c['motor_hole_z_position'], c['motor_hole_radius']))
+      six.print_(("ERR413: Error, motor_hole_z_position {:0.3f} is too small compare to motor_hole_radius {:0.3f}".format(c['motor_hole_z_position'], c['motor_hole_radius'])))
       sys.exit(2)
     if(c['motor_hole_z_position']>c['bell_face_height']+c['leg_length']-c['motor_hole_z_distance']-2*c['motor_hole_radius']-radian_epsilon):
-      print("ERR416: Error, motor_hole_z_distance {:0.3f} is too big compare to bell_face_height {:0.3f}, leg_length {:0.3f}, motor_hole_z_distance {:0.3f} and motor_hole_radius {:0.3f}".format(c['motor_hole_z_distance'], c['bell_face_height'], c['leg_length'], c['motor_hole_z_distance'], c['motor_hole_radius']))
+      six.print_(("ERR416: Error, motor_hole_z_distance {:0.3f} is too big compare to bell_face_height {:0.3f}, leg_length {:0.3f}, motor_hole_z_distance {:0.3f} and motor_hole_radius {:0.3f}".format(c['motor_hole_z_distance'], c['bell_face_height'], c['leg_length'], c['motor_hole_z_distance'], c['motor_hole_radius'])))
       sys.exit(2)
   # int_buttress_x_length
   if(c['int_buttress_x_length']>(c['bell_face_width']-2*c['side_thickness'])/3.0):
-    print("ERR426: Error, int_buttress_x_length {:0.3f} is too big compare to bell_face_width {:0.3f} and side_thickness {:0.3f}".format(c['int_buttress_x_length'], c['bell_face_width'], c['side_thickness']))
+    six.print_(("ERR426: Error, int_buttress_x_length {:0.3f} is too big compare to bell_face_width {:0.3f} and side_thickness {:0.3f}".format(c['int_buttress_x_length'], c['bell_face_width'], c['side_thickness'])))
     sys.exit(2)
   if(c['int_buttress_x_length']>0):
     # int_buttress_z_width
     if(c['int_buttress_z_width']<3*c['bell_cnc_router_bit_radius']):
-      print("ERR431: Error, int_buttress_z_width {:0.3f} is too small compare to bell_cnc_router_bit_radius {:0.3f}".format(c['int_buttress_z_width'], c['bell_cnc_router_bit_radius']))
+      six.print_(("ERR431: Error, int_buttress_z_width {:0.3f} is too small compare to bell_cnc_router_bit_radius {:0.3f}".format(c['int_buttress_z_width'], c['bell_cnc_router_bit_radius'])))
       sys.exit(2)
     if(c['int_buttress_z_width']>c['bell_face_height']/5.0):
-      print("ERR434: Error, int_buttress_z_width {:0.3f} is too big compare to bell_face_height {:0.3f}".format(c['int_buttress_z_width'], c['bell_face_height']))
+      six.print_(("ERR434: Error, int_buttress_z_width {:0.3f} is too big compare to bell_face_height {:0.3f}".format(c['int_buttress_z_width'], c['bell_face_height'])))
       sys.exit(2)
     # int_buttress_z_distance
     if(c['int_buttress_z_distance']<c['int_buttress_z_width']+radian_epsilon):
-      print("ERR438: Error, int_buttress_z_distance {:0.3f} must be bigger than int_buttress_z_width {:0.3f}".format(c['int_buttress_z_distance'], c['int_buttress_z_width']))
+      six.print_(("ERR438: Error, int_buttress_z_distance {:0.3f} must be bigger than int_buttress_z_width {:0.3f}".format(c['int_buttress_z_distance'], c['int_buttress_z_width'])))
       sys.exit(2)
     if(c['int_buttress_z_distance']>c['bell_face_height']-2*c['int_buttress_z_width']-radian_epsilon):
-      print("ERR441: Error, int_buttress_z_distance {:0.3f} is too big compare to bell_face_height {:0.3f} and int_buttress_z_width {:0.3f}".format(c['int_buttress_z_distance'], c['bell_face_height'], c['int_buttress_z_width']))
+      six.print_(("ERR441: Error, int_buttress_z_distance {:0.3f} is too big compare to bell_face_height {:0.3f} and int_buttress_z_width {:0.3f}".format(c['int_buttress_z_distance'], c['bell_face_height'], c['int_buttress_z_width'])))
       sys.exit(2)
     # int_buttress_x_position
     if(c['int_buttress_x_position']>c['bell_face_width']/2.0-c['face_thickness']-c['int_buttress_x_length']):
-      print("ERR445: Error, int_buttress_x_position {:0.3f} is too big compare to bell_face_width {:0.3f}, face_thickness {:0.3f} and int_buttress_x_length {:0.3f}".format(c['int_buttress_x_position'], c['bell_face_width'], c['face_thickness'], c['int_buttress_x_length']))
+      six.print_(("ERR445: Error, int_buttress_x_position {:0.3f} is too big compare to bell_face_width {:0.3f}, face_thickness {:0.3f} and int_buttress_x_length {:0.3f}".format(c['int_buttress_x_position'], c['bell_face_width'], c['face_thickness'], c['int_buttress_x_length'])))
       sys.exit(2)
     # int_buttress_z_position
     if(c['int_buttress_z_position']>c['bell_face_height']-c['int_buttress_z_distance']-c['int_buttress_z_width']-radian_epsilon):
-      print("ERR448: int_buttress_z_position {:0.3f} is too big compare to bell_face_height {:0.3f}, int_buttress_z_distance {:0.3f} and int_buttress_z_width {:0.3f}".format(c['int_buttress_z_position'], c['bell_face_height'], c['int_buttress_z_distance'], c['int_buttress_z_width']))
+      six.print_(("ERR448: int_buttress_z_position {:0.3f} is too big compare to bell_face_height {:0.3f}, int_buttress_z_distance {:0.3f} and int_buttress_z_width {:0.3f}".format(c['int_buttress_z_position'], c['bell_face_height'], c['int_buttress_z_distance'], c['int_buttress_z_width'])))
       sys.exit(2)
     # int_buttress_int_corner_length
     if(c['int_buttress_int_corner_length']>c['int_buttress_x_position']):
-      print("ERR453: Error, int_buttress_int_corner_length {:0.3f} must be smaller than int_buttress_x_position {:0.3f}".format(c['int_buttress_int_corner_length'], c['int_buttress_x_position']))
+      six.print_(("ERR453: Error, int_buttress_int_corner_length {:0.3f} must be smaller than int_buttress_x_position {:0.3f}".format(c['int_buttress_int_corner_length'], c['int_buttress_x_position'])))
       sys.exit(2)
     # int_buttress_ext_corner_length
     if(c['int_buttress_ext_corner_length']>c['bell_face_width']/2.0-c['face_thickness']-c['int_buttress_x_length']-c['int_buttress_x_position']):
-      print("ERR457: Error, int_buttress_ext_corner_length {:0.3f} is too big comapre to bell_face_width {:0.3f}, face_thickness {:0.3f}, int_buttress_x_length {:0.3f} and int_buttress_x_position {:0.3f}".format(c['int_buttress_ext_corner_length'], c['bell_face_width'], c['face_thickness'], c['int_buttress_x_length'], c['int_buttress_x_position']))
+      six.print_(("ERR457: Error, int_buttress_ext_corner_length {:0.3f} is too big comapre to bell_face_width {:0.3f}, face_thickness {:0.3f}, int_buttress_x_length {:0.3f} and int_buttress_x_position {:0.3f}".format(c['int_buttress_ext_corner_length'], c['bell_face_width'], c['face_thickness'], c['int_buttress_x_length'], c['int_buttress_x_position'])))
       sys.exit(2)
     # int_buttress_bump_length
     if(c['int_buttress_bump_length']>c['int_buttress_x_position']+c['int_buttress_x_length']):
-      print("ERR461: Error, int_buttress_bump_length {:0.3f} is too big compare to int_buttress_x_position {:0.3f} and int_buttress_x_length {:0.3f}".format(c['int_buttress_bump_length'], c['int_buttress_x_position'], c['int_buttress_x_length']))
+      six.print_(("ERR461: Error, int_buttress_bump_length {:0.3f} is too big compare to int_buttress_x_position {:0.3f} and int_buttress_x_length {:0.3f}".format(c['int_buttress_bump_length'], c['int_buttress_x_position'], c['int_buttress_x_length'])))
       sys.exit(2)
     # int_buttress_arc_height
     if(abs(c['int_buttress_arc_height'])>c['int_buttress_x_length']):
-      print("ERR465: Error, int_buttress_arc_height {:0.3f} absolute value must be smaller than int_buttress_x_length {:0.3f}".format(c['int_buttress_arc_height'], c['int_buttress_x_length']))
+      six.print_(("ERR465: Error, int_buttress_arc_height {:0.3f} absolute value must be smaller than int_buttress_x_length {:0.3f}".format(c['int_buttress_arc_height'], c['int_buttress_x_length'])))
       sys.exit(2)
     # int_buttress_smoothing_radius
     if(c['int_buttress_smoothing_radius']>c['int_buttress_bump_length']):
-      print("ERR469: Error, int_buttress_smoothing_radius {:0.3f} must be smaller than int_buttress_bump_length {:0.3f}".format(c['int_buttress_smoothing_radius'], c['int_buttress_bump_length']))
+      six.print_(("ERR469: Error, int_buttress_smoothing_radius {:0.3f} must be smaller than int_buttress_bump_length {:0.3f}".format(c['int_buttress_smoothing_radius'], c['int_buttress_bump_length'])))
       sys.exit(2)
   # ext_buttress_z_length
   if(c['ext_buttress_z_length']>c['bell_face_height']/3.0):
-    print("ERR473: Error, ext_buttress_z_length {:0.3f} is too big compare to bell_face_height {:0.3f}".format(c['ext_buttress_z_length'], c['bell_face_height']))
+    six.print_(("ERR473: Error, ext_buttress_z_length {:0.3f} is too big compare to bell_face_height {:0.3f}".format(c['ext_buttress_z_length'], c['bell_face_height'])))
     sys.exit(2)
   if(c['ext_buttress_z_length']>0):
     # ext_buttress_x_width
     if(c['ext_buttress_x_width']<3*c['bell_cnc_router_bit_radius']):
-      print("ERR478: Error, ext_buttress_x_width {:0.3f} is too small compare to bell_cnc_router_bit_radius {:0.3f}".format(c['ext_buttress_x_width'], c['bell_cnc_router_bit_radius']))
+      six.print_(("ERR478: Error, ext_buttress_x_width {:0.3f} is too small compare to bell_cnc_router_bit_radius {:0.3f}".format(c['ext_buttress_x_width'], c['bell_cnc_router_bit_radius'])))
       sys.exit(2)
     if(c['ext_buttress_x_width']>c['bell_face_width']/5.0):
-      print("ERR481: Error, ext_buttress_x_width {:0.3f} is too big compare to bell_face_width {:0.3f}".format(c['ext_buttress_x_width'], c['bell_face_width']))
+      six.print_(("ERR481: Error, ext_buttress_x_width {:0.3f} is too big compare to bell_face_width {:0.3f}".format(c['ext_buttress_x_width'], c['bell_face_width'])))
       sys.exit(2)
     # ext_buttress_x_distance
     if(c['ext_buttress_x_distance']<radian_epsilon):
-      print("ERR485: Error, ext_buttress_x_distance {:0.3f} must be strictly positive".format(c['ext_buttress_x_distance']))
+      six.print_(("ERR485: Error, ext_buttress_x_distance {:0.3f} must be strictly positive".format(c['ext_buttress_x_distance'])))
       sys.exit(2)
     if(c['ext_buttress_x_distance']>c['bell_face_width']-2*c['ext_buttress_x_width']-2*c['face_thickness']):
-      print("ERR488: Error, ext_buttress_x_distance {:0.3f} is too big compare to bell_face_width {:0.3f} ext_buttress_x_width {:0.3f} and face_thickness {:0.3f}".format(c['ext_buttress_x_distance'], c['bell_face_width'], c['ext_buttress_x_width'], c['face_thickness']))
+      six.print_(("ERR488: Error, ext_buttress_x_distance {:0.3f} is too big compare to bell_face_width {:0.3f} ext_buttress_x_width {:0.3f} and face_thickness {:0.3f}".format(c['ext_buttress_x_distance'], c['bell_face_width'], c['ext_buttress_x_width'], c['face_thickness'])))
       sys.exit(2)
     # ext_buttress_z_position
     if(c['ext_buttress_z_position']>c['bell_face_height']+c['leg_length']-c['ext_buttress_z_length']):
-      print("ERR490: Error, ext_buttress_z_position {:0.3f} is too big compare to bell_face_height {:0.3f}, leg_length {:0.3f} and ext_buttress_z_length {:0.3f}".format(c['ext_buttress_z_position'], c['bell_face_height'], c['leg_length'], c['ext_buttress_z_length']))
+      six.print_(("ERR490: Error, ext_buttress_z_position {:0.3f} is too big compare to bell_face_height {:0.3f}, leg_length {:0.3f} and ext_buttress_z_length {:0.3f}".format(c['ext_buttress_z_position'], c['bell_face_height'], c['leg_length'], c['ext_buttress_z_length'])))
       sys.exit(2)
     # ext_buttress_y_length
     if(c['ext_buttress_y_length']>c['base_radius']-c['bell_face_width']/2.0):
-      print("ERR499: Error, ext_buttress_y_length {:0.3f} is too big compare to base_radius {:0.3f} and bell_face_width {:0.3f}".format(c['ext_buttress_y_length'], c['base_radius'], c['bell_face_width']))
+      six.print_(("ERR499: Error, ext_buttress_y_length {:0.3f} is too big compare to base_radius {:0.3f} and bell_face_width {:0.3f}".format(c['ext_buttress_y_length'], c['base_radius'], c['bell_face_width'])))
       sys.exit(2)
     # ext_buttress_y_position
     if(math.sqrt((c['ext_buttress_x_distance']/2.0+c['ext_buttress_x_width'])**2+(c['bell_face_width']/2.0+c['ext_buttress_y_position']+c['ext_buttress_y_length'])**2)>c['base_radius']):
-      print("ERR502: Error, ext_buttress_y_position {:0.3f} is too big compare to ext_buttress_x_distance {:0.3f}, ext_buttress_x_width {:0.3f}, bell_face_width {:0.3f}, ext_buttress_y_length {:0.3f} and base_radius {:0.3f}".format(c['ext_buttress_y_position'], c['ext_buttress_x_distance'], c['ext_buttress_x_width'], c['bell_face_width'], c['ext_buttress_y_length'], c['base_radius']))
+      six.print_(("ERR502: Error, ext_buttress_y_position {:0.3f} is too big compare to ext_buttress_x_distance {:0.3f}, ext_buttress_x_width {:0.3f}, bell_face_width {:0.3f}, ext_buttress_y_length {:0.3f} and base_radius {:0.3f}".format(c['ext_buttress_y_position'], c['ext_buttress_x_distance'], c['ext_buttress_x_width'], c['bell_face_width'], c['ext_buttress_y_length'], c['base_radius'])))
       sys.exit(2)
     # ext_buttress_face_int_corner_length
     if(c['ext_buttress_face_int_corner_length']>c['ext_buttress_z_position']):
-      print("ERR507: Error, ext_buttress_face_int_corner_length {:0.3f} must be smaller than ext_buttress_z_position {:0.3f}".format(c['ext_buttress_face_int_corner_length'], c['ext_buttress_z_position']))
+      six.print_(("ERR507: Error, ext_buttress_face_int_corner_length {:0.3f} must be smaller than ext_buttress_z_position {:0.3f}".format(c['ext_buttress_face_int_corner_length'], c['ext_buttress_z_position'])))
       sys.exit(2)
     # ext_buttress_face_ext_corner_length
     if(c['ext_buttress_face_ext_corner_length']>c['bell_face_height']+c['leg_length']-c['ext_buttress_z_length']):
-      print("ext_buttress_face_ext_corner_length {:0.3f} is too big compare to bell_face_height {:0.3f}, leg_length {:0.3f} and ext_buttress_z_length {:0.3f}".format(c['ext_buttress_face_ext_corner_length'], c['bell_face_height'], c['leg_length'], c['ext_buttress_z_length']))
+      six.print_(("ext_buttress_face_ext_corner_length {:0.3f} is too big compare to bell_face_height {:0.3f}, leg_length {:0.3f} and ext_buttress_z_length {:0.3f}".format(c['ext_buttress_face_ext_corner_length'], c['bell_face_height'], c['leg_length'], c['ext_buttress_z_length'])))
       sys.exit(2)
     # ext_buttress_face_bump_length
     if(c['ext_buttress_face_bump_length']>c['ext_buttress_y_position']+c['ext_buttress_y_length']):
-      print("ERR515: Error, ext_buttress_face_bump_length {:0.3f} is too big compare to ext_buttress_y_position {:0.3f} and ext_buttress_y_length {:0.3f}".format(c['ext_buttress_face_bump_length'], c['ext_buttress_y_position'], c['ext_buttress_y_length']))
+      six.print_(("ERR515: Error, ext_buttress_face_bump_length {:0.3f} is too big compare to ext_buttress_y_position {:0.3f} and ext_buttress_y_length {:0.3f}".format(c['ext_buttress_face_bump_length'], c['ext_buttress_y_position'], c['ext_buttress_y_length'])))
       sys.exit(2)
     # ext_buttress_base_int_corner_length
     if(c['ext_buttress_base_int_corner_length']>c['ext_buttress_y_position']):
-      print("ERR519: Error, ext_buttress_base_int_corner_length {:0.3f} must be smaller than ext_buttress_y_position {:0.3f}".format(c['ext_buttress_base_int_corner_length'], c['ext_buttress_y_position']))
+      six.print_(("ERR519: Error, ext_buttress_base_int_corner_length {:0.3f} must be smaller than ext_buttress_y_position {:0.3f}".format(c['ext_buttress_base_int_corner_length'], c['ext_buttress_y_position'])))
       sys.exit(2)
     # ext_buttress_base_ext_corner_length
     if(c['ext_buttress_base_ext_corner_length']>c['base_radius']-(c['bell_face_width']/2.0+c['ext_buttress_y_position']+c['ext_buttress_y_length'])):
-      print("ERR523: Error, ext_buttress_base_ext_corner_length {:0.3f} is too big compare to base_radius {:0.3f}, bell_face_width {:0.3f}, ext_buttress_y_position {:0.3f} and ext_buttress_y_length {:0.3f}".format(c['ext_buttress_base_ext_corner_length'], c['base_radius'], c['bell_face_width'], c['ext_buttress_y_position'], c['ext_buttress_y_length']))
+      six.print_(("ERR523: Error, ext_buttress_base_ext_corner_length {:0.3f} is too big compare to base_radius {:0.3f}, bell_face_width {:0.3f}, ext_buttress_y_position {:0.3f} and ext_buttress_y_length {:0.3f}".format(c['ext_buttress_base_ext_corner_length'], c['base_radius'], c['bell_face_width'], c['ext_buttress_y_position'], c['ext_buttress_y_length'])))
       sys.exit(2)
     # ext_buttress_base_bump_length
     if(c['ext_buttress_base_bump_length']>c['ext_buttress_z_position']+c['ext_buttress_z_length']):
-      print("ERR527: Error, ext_buttress_base_bump_length {:0.3f} is too big compare to ext_buttress_z_position {:0.3f} and ext_buttress_z_length {:0.3f}".format(c['ext_buttress_base_bump_length'], c['ext_buttress_z_position'], c['ext_buttress_z_length']))
+      six.print_(("ERR527: Error, ext_buttress_base_bump_length {:0.3f} is too big compare to ext_buttress_z_position {:0.3f} and ext_buttress_z_length {:0.3f}".format(c['ext_buttress_base_bump_length'], c['ext_buttress_z_position'], c['ext_buttress_z_length'])))
       sys.exit(2)
     # ext_buttress_arc_height
     if(abs(c['ext_buttress_arc_height'])>max(c['ext_buttress_z_length'], c['ext_buttress_y_length'])):
-      print("ERR531: Error, ext_buttress_arc_height {:0.3f} absolute value is too big compare to ext_buttress_z_length {:0.3f} and ext_buttress_y_length {:0.3f}".format(c['ext_buttress_arc_height'], c['ext_buttress_z_length'], c['ext_buttress_y_length']))
+      six.print_(("ERR531: Error, ext_buttress_arc_height {:0.3f} absolute value is too big compare to ext_buttress_z_length {:0.3f} and ext_buttress_y_length {:0.3f}".format(c['ext_buttress_arc_height'], c['ext_buttress_z_length'], c['ext_buttress_y_length'])))
       sys.exit(2)
     # ext_buttress_smoothing_radius
     if(c['ext_buttress_smoothing_radius']>max(c['ext_buttress_face_bump_length'], c['ext_buttress_base_bump_length'])):
-      print("ERR535: Error, ext_buttress_smoothing_radius {:0.3f} is too big compare to ext_buttress_face_bump_length {:0.3f} and ext_buttress_base_bump_length {:0.3f}".format(c['ext_buttress_smoothing_radius'], c['ext_buttress_face_bump_length'], c['ext_buttress_base_bump_length']))
+      six.print_(("ERR535: Error, ext_buttress_smoothing_radius {:0.3f} is too big compare to ext_buttress_face_bump_length {:0.3f} and ext_buttress_base_bump_length {:0.3f}".format(c['ext_buttress_smoothing_radius'], c['ext_buttress_face_bump_length'], c['ext_buttress_base_bump_length'])))
       sys.exit(2)
   # hollow_z_height
   if(c['hollow_z_height']>c['bell_face_height']):
-    print("ERR539: Error, hollow_z_height {:0.3f} must be smaller than bell_face_height {:0.3f}".format(c['hollow_z_height'], c['bell_face_height']))
+    six.print_(("ERR539: Error, hollow_z_height {:0.3f} must be smaller than bell_face_height {:0.3f}".format(c['hollow_z_height'], c['bell_face_height'])))
     sys.exit(2)
   # hollow_y_width
   if(c['hollow_y_width']<2*c['bell_cnc_router_bit_radius']):
-    print("ERR543: Error, hollow_y_width {:0.3f} is too small compare to bell_cnc_router_bit_radius {:0.3f}".format(c['hollow_y_width'], c['bell_cnc_router_bit_radius']))
+    six.print_(("ERR543: Error, hollow_y_width {:0.3f} is too small compare to bell_cnc_router_bit_radius {:0.3f}".format(c['hollow_y_width'], c['bell_cnc_router_bit_radius'])))
     sys.exit(2)
   if(c['hollow_y_width']>c['bell_face_width']-2*c['face_thickness']):
-    print("ERR546: Error, hollow_y_width {:0.3f} is too big compare to bell_face_width {:0.3f} and face_thickness {:0.3f}".format(c['hollow_y_width'], c['bell_face_width'], c['face_thickness']))
+    six.print_(("ERR546: Error, hollow_y_width {:0.3f} is too big compare to bell_face_width {:0.3f} and face_thickness {:0.3f}".format(c['hollow_y_width'], c['bell_face_width'], c['face_thickness'])))
     sys.exit(2)
   # hollow_spare_width
   if(c['hollow_spare_width']>c['bell_face_width']/2-c['face_thickness']-c['hollow_y_width']/2.0):
-    print("ERR550: Error, hollow_spare_width {:0.3f} is too big compare to bell_face_width {:0.3f}, face_thickness {:0.3f} and hollow_y_width {:0.3f}".format(c['hollow_spare_width'], c['bell_face_width'], c['face_thickness'], c['hollow_y_width']))
+    six.print_(("ERR550: Error, hollow_spare_width {:0.3f} is too big compare to bell_face_width {:0.3f}, face_thickness {:0.3f} and hollow_y_width {:0.3f}".format(c['hollow_spare_width'], c['bell_face_width'], c['face_thickness'], c['hollow_y_width'])))
     sys.exit(2)
   # base_hole_nb
   if(c['base_hole_nb']>0):
     # base_hole_diameter
     c['base_hole_radius'] = c['base_hole_diameter']/2.0
     if(c['base_hole_radius']<radian_epsilon):
-      print("ERR556: Error, base_hole_radius {:0.3f} must be strictly positive".format(c['base_hole_radius']))
+      six.print_(("ERR556: Error, base_hole_radius {:0.3f} must be strictly positive".format(c['base_hole_radius'])))
       sys.exit(2)
     # base_hole_position_diameter
     c['base_hole_position_radius'] = c['base_hole_position_diameter']/2.0
     if(c['base_hole_position_radius']==0):
       c['base_hole_position_radius'] = c['base_radius'] - 2*c['base_hole_radius']
     if(c['base_hole_position_radius']<c['bell_face_width']/2.0):
-      print("ERR564: Error, base_hole_position_radius {:0.3f} is too small compare to bell_face_width {:0.3f}".format(c['base_hole_position_radius'], c['bell_face_width']))
+      six.print_(("ERR564: Error, base_hole_position_radius {:0.3f} is too small compare to bell_face_width {:0.3f}".format(c['base_hole_position_radius'], c['bell_face_width'])))
       sys.exit(2)
     if(c['base_hole_position_radius']>c['base_radius']-c['base_hole_radius']):
-      print("ERR567: Error, base_hole_position_radius {:0.3f} is too big compare to base_radius {:0.3f} and base_hole_radius {:0.3f}".format(c['base_hole_position_radius'], c['base_radius'], c['base_hole_radius']))
+      six.print_(("ERR567: Error, base_hole_position_radius {:0.3f} is too big compare to base_radius {:0.3f} and base_hole_radius {:0.3f}".format(c['base_hole_position_radius'], c['base_radius'], c['base_hole_radius'])))
       sys.exit(2)
     # base_hole_angle
   # y_hole_diameter
@@ -471,68 +472,68 @@ def bell_constraint_check(c):
   if(c['y_hole_radius']>0):
     # y_hole_z_top_position
     if(abs(c['y_hole_z_top_position'])>c['bell_face_height']):
-      print("ERR575: Error, y_hole_z_top_position {:0.3f} absolute value is too big compare to bell_face_height {:0.3f}".format(c['y_hole_z_top_position'], c['bell_face_height']))
+      six.print_(("ERR575: Error, y_hole_z_top_position {:0.3f} absolute value is too big compare to bell_face_height {:0.3f}".format(c['y_hole_z_top_position'], c['bell_face_height'])))
       sys.exit(2)
     if(c['y_hole_z_top_position']>0):
       if(c['y_hole_z_top_position']<c['int_buttress_z_width']+c['y_hole_radius']):
-        print("ERR580: Error, positive y_hole_z_top_position {:0.3f} is too small compare to int_buttress_z_width {:0.3f} and y_hole_radius {:0.3f}".format(c['y_hole_z_top_position'], c['int_buttress_z_width'], c['y_hole_radius']))
+        six.print_(("ERR580: Error, positive y_hole_z_top_position {:0.3f} is too small compare to int_buttress_z_width {:0.3f} and y_hole_radius {:0.3f}".format(c['y_hole_z_top_position'], c['int_buttress_z_width'], c['y_hole_radius'])))
         sys.exit(2)
     else:
       if(c['y_hole_z_top_position']>-1*c['y_hole_radius']):
-        print("ERR584: Error, negative y_hole_z_top_position {:0.3f} must be smaller than y_hole_radius {:0.3f}".format(c['y_hole_z_top_position'], -1*c['y_hole_radius']))
+        six.print_(("ERR584: Error, negative y_hole_z_top_position {:0.3f} must be smaller than y_hole_radius {:0.3f}".format(c['y_hole_z_top_position'], -1*c['y_hole_radius'])))
         sys.exit(2)
     # y_hole_z_bottom_position
     if(abs(c['y_hole_z_bottom_position'])>c['bell_face_height']):
-      print("ERR575: Error, y_hole_z_bottom_position {:0.3f} absolute value is too big compare to bell_face_height {:0.3f}".format(c['y_hole_z_bottom_position'], c['bell_face_height']))
+      six.print_(("ERR575: Error, y_hole_z_bottom_position {:0.3f} absolute value is too big compare to bell_face_height {:0.3f}".format(c['y_hole_z_bottom_position'], c['bell_face_height'])))
       sys.exit(2)
     if(c['y_hole_z_bottom_position']>0):
       if(c['y_hole_z_bottom_position']<c['int_buttress_z_width']+c['y_hole_radius']):
-        print("ERR580: Error, positive y_hole_z_bottom_position {:0.3f} is too small compare to int_buttress_z_width {:0.3f} and y_hole_radius {:0.3f}".format(c['y_hole_z_bottom_position'], c['int_buttress_z_width'], c['y_hole_radius']))
+        six.print_(("ERR580: Error, positive y_hole_z_bottom_position {:0.3f} is too small compare to int_buttress_z_width {:0.3f} and y_hole_radius {:0.3f}".format(c['y_hole_z_bottom_position'], c['int_buttress_z_width'], c['y_hole_radius'])))
         sys.exit(2)
     else:
       if(c['y_hole_z_bottom_position']>-1*c['y_hole_radius']):
-        print("ERR584: Error, negative y_hole_z_bottom_position {:0.3f} must be smaller than y_hole_radius {:0.3f}".format(c['y_hole_z_bottom_position'], -1*c['y_hole_radius']))
+        six.print_(("ERR584: Error, negative y_hole_z_bottom_position {:0.3f} must be smaller than y_hole_radius {:0.3f}".format(c['y_hole_z_bottom_position'], -1*c['y_hole_radius'])))
         sys.exit(2)
     # y_hole_x_position
     if(c['y_hole_x_position']<c['y_hole_radius']):
-      print("ERR588: Error, y_hole_x_position {:0.3f} is too small compare to y_hole_radius {:0.3f}".format(c['y_hole_x_position'], c['y_hole_radius']))
+      six.print_(("ERR588: Error, y_hole_x_position {:0.3f} is too small compare to y_hole_radius {:0.3f}".format(c['y_hole_x_position'], c['y_hole_radius'])))
       sys.exit(2)
     if(c['y_hole_x_position']>c['bell_face_width']/2.0-c['side_thickness']):
-      print("ERR591: Error, y_hole_x_position {:0.3f} is too big compare to bell_face_width {:0.3f} and side_thickness {:0.3f}".format(c['y_hole_x_position'], c['bell_face_width'], c['side_thickness']))
+      six.print_(("ERR591: Error, y_hole_x_position {:0.3f} is too big compare to bell_face_width {:0.3f} and side_thickness {:0.3f}".format(c['y_hole_x_position'], c['bell_face_width'], c['side_thickness'])))
       sys.exit(2)
   # x_hole_diameter
   c['x_hole_radius'] = c['x_hole_diameter']/2.0
   if(c['x_hole_radius']>0):
     # x_hole_z_top_position
     if(abs(c['x_hole_z_top_position'])>c['bell_face_height']):
-      print("ERR598: Error, x_hole_z_top_position {:0.3f} absolute value is too big compare to bell_face_height {:0.3f}".format(c['x_hole_z_top_position'], c['bell_face_height']))
+      six.print_(("ERR598: Error, x_hole_z_top_position {:0.3f} absolute value is too big compare to bell_face_height {:0.3f}".format(c['x_hole_z_top_position'], c['bell_face_height'])))
       sys.exit(2)
     if(c['x_hole_z_top_position']>0):
       if(c['x_hole_z_top_position']<c['int_buttress_z_width']+c['x_hole_radius']):
-        print("ERR580: Error, positive x_hole_z_top_position {:0.3f} is too small compare to int_buttress_z_width {:0.3f} and x_hole_radius {:0.3f}".format(c['x_hole_z_top_position'], c['int_buttress_z_width'], c['x_hole_radius']))
+        six.print_(("ERR580: Error, positive x_hole_z_top_position {:0.3f} is too small compare to int_buttress_z_width {:0.3f} and x_hole_radius {:0.3f}".format(c['x_hole_z_top_position'], c['int_buttress_z_width'], c['x_hole_radius'])))
         sys.exit(2)
     else:
       if(c['x_hole_z_top_position']>-1*c['x_hole_radius']):
-        print("ERR584: Error, negative x_hole_z_top_position {:0.3f} must be smaller than x_hole_radius {:0.3f}".format(c['x_hole_z_top_position'], -1*c['x_hole_radius']))
+        six.print_(("ERR584: Error, negative x_hole_z_top_position {:0.3f} must be smaller than x_hole_radius {:0.3f}".format(c['x_hole_z_top_position'], -1*c['x_hole_radius'])))
         sys.exit(2)
     # x_hole_z_bottom_position
     if(abs(c['x_hole_z_bottom_position'])>c['bell_face_height']):
-      print("ERR598: Error, x_hole_z_bottom_position {:0.3f} absolute value is too big compare to bell_face_height {:0.3f}".format(c['x_hole_z_bottom_position'], c['bell_face_height']))
+      six.print_(("ERR598: Error, x_hole_z_bottom_position {:0.3f} absolute value is too big compare to bell_face_height {:0.3f}".format(c['x_hole_z_bottom_position'], c['bell_face_height'])))
       sys.exit(2)
     if(c['x_hole_z_bottom_position']>0):
       if(c['x_hole_z_bottom_position']<c['int_buttress_z_width']+c['x_hole_radius']):
-        print("ERR580: Error, positive x_hole_z_bottom_position {:0.3f} is too small compare to int_buttress_z_width {:0.3f} and x_hole_radius {:0.3f}".format(c['x_hole_z_bottom_position'], c['int_buttress_z_width'], c['x_hole_radius']))
+        six.print_(("ERR580: Error, positive x_hole_z_bottom_position {:0.3f} is too small compare to int_buttress_z_width {:0.3f} and x_hole_radius {:0.3f}".format(c['x_hole_z_bottom_position'], c['int_buttress_z_width'], c['x_hole_radius'])))
         sys.exit(2)
     else:
       if(c['x_hole_z_bottom_position']>-1*c['x_hole_radius']):
-        print("ERR584: Error, negative x_hole_z_bottom_position {:0.3f} must be smaller than x_hole_radius {:0.3f}".format(c['x_hole_z_bottom_position'], -1*c['x_hole_radius']))
+        six.print_(("ERR584: Error, negative x_hole_z_bottom_position {:0.3f} must be smaller than x_hole_radius {:0.3f}".format(c['x_hole_z_bottom_position'], -1*c['x_hole_radius'])))
         sys.exit(2)
     # x_hole_y_position
     if(c['x_hole_y_position']<c['x_hole_radius']):
-      print("ERR588: Error, x_hole_y_position {:0.3f} is too small compare to x_hole_radius {:0.3f}".format(c['x_hole_y_position'], c['x_hole_radius']))
+      six.print_(("ERR588: Error, x_hole_y_position {:0.3f} is too small compare to x_hole_radius {:0.3f}".format(c['x_hole_y_position'], c['x_hole_radius'])))
       sys.exit(2)
     if(c['x_hole_y_position']>c['bell_face_width']/2.0-c['side_thickness']):
-      print("ERR591: Error, x_hole_y_position {:0.3f} is too big compare to bell_face_width {:0.3f} and side_thickness {:0.3f}".format(c['x_hole_y_position'], c['bell_face_width'], c['side_thickness']))
+      six.print_(("ERR591: Error, x_hole_y_position {:0.3f} is too big compare to bell_face_width {:0.3f} and side_thickness {:0.3f}".format(c['x_hole_y_position'], c['bell_face_width'], c['side_thickness'])))
       sys.exit(2)
   # z_hole_diameter
   c['z_hole_radius'] = c['z_hole_diameter']/2.0
@@ -542,18 +543,18 @@ def bell_constraint_check(c):
     c['z_hole_external_radius'] = 2*c['z_hole_radius']
   # z_hole_position_length
   if(c['z_hole_position_length']<math.sqrt(2)*c['z_hole_radius']):
-    print("ERR623: Error, z_hole_position_length {:0.3f} is too small compare to z_hole_radius {:0.3f}".format(c['z_hole_position_length'], c['z_hole_radius']))
+    six.print_(("ERR623: Error, z_hole_position_length {:0.3f} is too small compare to z_hole_radius {:0.3f}".format(c['z_hole_position_length'], c['z_hole_radius'])))
     sys.exit(2)
   if(c['z_hole_position_length']>c['bell_face_width']/2.0):
-    print("ERR626: Error, z_hole_position_length {:0.3f} is too big compare to bell_face_width {:0.3f}".format(c['z_hole_position_length'], c['bell_face_width']))
+    six.print_(("ERR626: Error, z_hole_position_length {:0.3f} is too big compare to bell_face_width {:0.3f}".format(c['z_hole_position_length'], c['bell_face_width'])))
     sys.exit(2)
   # bell_cnc_router_bit_radius
   if(c['bell_cnc_router_bit_radius']>min(c['face_thickness'], c['side_thickness'], c['base_thickness'])/2.0):
-    print("ERR634: Error, bell_cnc_router_bit_radius {:0.3f} is too big compare to face_thickness {:0.3f}, side_thickness {:0.3f} or base_thickness {:0.3f}".format(c['bell_cnc_router_bit_radius'], c['face_thickness'], c['side_thickness'], c['base_thickness']))
+    six.print_(("ERR634: Error, bell_cnc_router_bit_radius {:0.3f} is too big compare to face_thickness {:0.3f}, side_thickness {:0.3f} or base_thickness {:0.3f}".format(c['bell_cnc_router_bit_radius'], c['face_thickness'], c['side_thickness'], c['base_thickness'])))
     sys.exit(2)
   # bell_extra_cut_thickness
   if(c['bell_extra_cut_thickness']>min(c['bell_face_width'], c['bell_face_height'])/10.0):
-    print("ERR630: Error, bell_extra_cut_thickness {:0.3f} is too big compare to bell_face_width {:0.3f} and bell_face_height {:0.3f}".format(c['bell_extra_cut_thickness'], c['bell_face_width'], c['bell_face_height']))
+    six.print_(("ERR630: Error, bell_extra_cut_thickness {:0.3f} is too big compare to bell_face_width {:0.3f} and bell_face_height {:0.3f}".format(c['bell_extra_cut_thickness'], c['bell_face_width'], c['bell_face_height'])))
     sys.exit(2)
   ## intermediate parameters
   c['f_w2'] = c['bell_face_width']/2.0
@@ -896,7 +897,7 @@ def bell_cli_return_type(return_type, c):
   elif(return_type=='figures_3dconf_info'):
     r_b = (figures['part_list'], assembly_conf['bell_assembly_conf1'], b_info)
   else:
-    print("ERR508: Error the return_type {:s} is unknown".format(return_type))
+    six.print_(("ERR508: Error the return_type {:s} is unknown".format(return_type)))
     sys.exit(2)
   return(r_b)
 
diff --git a/cnc25d/bell_bagel_assembly.py b/cnc25d/bell_bagel_assembly.py
index 5e25bf5..f952fe4 100644
--- a/cnc25d/bell_bagel_assembly.py
+++ b/cnc25d/bell_bagel_assembly.py
@@ -29,7 +29,8 @@
 # header for Python / FreeCAD compatibility
 ################################################################
 
-import cnc25d_api
+from . import cnc25d_api
+import six
 cnc25d_api.importing_freecad()
 
 #print("FreeCAD.Version:", FreeCAD.Version())
@@ -52,8 +53,8 @@
 #import svgwrite
 #from dxfwrite import DXFEngine
 # cnc25d
-import bell
-import bagel
+from . import bell
+from . import bagel
 
 ################################################################
 # inheritance from designs bell and bagel
@@ -117,7 +118,7 @@ def bba_constraint_check(c):
   # bagel_axle_diameter
   c['bagel_axle_radius'] = c['bagel_axle_diameter']/2.0
   if(c['bagel_axle_radius']<radian_epsilon):
-    print("ERR125: Error, bagel_axle_radius {:0.3f} is too small".format(c['bagel_axle_radius']))
+    six.print_(("ERR125: Error, bagel_axle_radius {:0.3f} is too small".format(c['bagel_axle_radius'])))
     sys.exit(2)
   c['bagel_axle_diameter'] = 2*c['bagel_axle_radius']
   # bagel_axle_internal_diameter
@@ -125,7 +126,7 @@ def bba_constraint_check(c):
   if(c['bagel_axle_internal_radius']==0):
     c['bagel_axle_internal_radius'] = 2*c['bagel_axle_radius']
   if(c['bagel_axle_internal_radius']<c['bagel_axle_radius']):
-    print("ERR132: Error, bagel_axle_internal_radius {:0.3f} must be bigger than bagel_axle_radius {:0.3f}".format(c['bagel_axle_internal_radius'], c['bagel_axle_radius']))
+    six.print_(("ERR132: Error, bagel_axle_internal_radius {:0.3f} must be bigger than bagel_axle_radius {:0.3f}".format(c['bagel_axle_internal_radius'], c['bagel_axle_radius'])))
     sys.exit(2)
   c['bagel_axle_internal_diameter'] = 2*c['bagel_axle_internal_radius']
   # bagel_axle_external_diameter
@@ -133,7 +134,7 @@ def bba_constraint_check(c):
   if(c['bagel_axle_external_radius']==0):
     c['bagel_axle_external_radius'] = 2*c['bagel_axle_internal_radius']
   if(c['bagel_axle_external_radius']<c['bagel_axle_internal_radius']+radian_epsilon):
-    print("ERR139: Error, bagel_axle_external_radius {:0.3f} must be bigger than bagel_axle_internal_radius {:0.3f}".format(c['bagel_axle_external_radius'], c['bagel_axle_internal_radius']))
+    six.print_(("ERR139: Error, bagel_axle_external_radius {:0.3f} must be bigger than bagel_axle_internal_radius {:0.3f}".format(c['bagel_axle_external_radius'], c['bagel_axle_internal_radius'])))
     sys.exit(2)
   c['bagel_axle_external_diameter'] = 2*c['bagel_axle_external_radius']
   # axle_internal_diameter
@@ -141,7 +142,7 @@ def bba_constraint_check(c):
   if(c['axle_internal_radius']==0):
     c['axle_internal_radius'] = c['bagel_axle_internal_radius']
   if(c['axle_internal_radius']<c['bagel_axle_radius']):
-    print("ERR146: Error, axle_internal_radius {:0.3f} must be bigger than bagel_axle_radius {:0.3f}".format(c['axle_internal_radius'], c['bagel_axle_radius']))
+    six.print_(("ERR146: Error, axle_internal_radius {:0.3f} must be bigger than bagel_axle_radius {:0.3f}".format(c['axle_internal_radius'], c['bagel_axle_radius'])))
     sys.exit(2)
   c['axle_internal_diameter'] = 2*c['axle_internal_radius']
   # axle_external_diameter
@@ -149,7 +150,7 @@ def bba_constraint_check(c):
   if(c['axle_external_radius']==0):
     c['axle_external_radius'] = 2*c['axle_internal_radius']
   if(c['axle_external_radius']<c['axle_internal_radius']+radian_epsilon):
-    print("ERR139: Error, axle_external_radius {:0.3f} must be bigger than axle_internal_radius {:0.3f}".format(c['axle_external_radius'], c['axle_internal_radius']))
+    six.print_(("ERR139: Error, axle_external_radius {:0.3f} must be bigger than axle_internal_radius {:0.3f}".format(c['axle_external_radius'], c['axle_internal_radius'])))
     sys.exit(2)
   c['axle_external_diameter'] = 2*c['axle_external_radius']
   ### sub-design check
diff --git a/cnc25d/bell_outline.py b/cnc25d/bell_outline.py
index 1a44035..c9b2f27 100644
--- a/cnc25d/bell_outline.py
+++ b/cnc25d/bell_outline.py
@@ -29,7 +29,7 @@
 # header for Python / FreeCAD compatibility
 ################################################################
 
-import cnc25d_api
+from . import cnc25d_api
 #cnc25d_api.importing_freecad()
 
 #print("FreeCAD.Version:", FreeCAD.Version())
@@ -47,7 +47,8 @@
 #from FreeCAD import Base
 #
 # cnc25d
-import small_geometry
+from . import small_geometry
+import six
 
 
 ################################################################
@@ -134,8 +135,8 @@ def bell_face_outline(ai_c):
   lAC = math.sqrt(lAB**2+lBC**2)
   cos_aBAC = float(lAB)/lAC
   if(abs(cos_aBAC)>1):
-    print("ERR063: Error, cos_aBAC {:0.3f} is out of the range -1..1".format(cos_aBAC))
-    print("dbg064: lAC {:0.3f}  lAB {:0.3f}".format(lAC, lAB))
+    six.print_(("ERR063: Error, cos_aBAC {:0.3f} is out of the range -1..1".format(cos_aBAC)))
+    six.print_(("dbg064: lAC {:0.3f}  lAB {:0.3f}".format(lAC, lAB)))
     sys.exit(2)
   aBAC = math.acos(cos_aBAC)
   #lAD = math.sqrt(lAC**2-lCD**2)
@@ -166,7 +167,7 @@ def bell_face_outline(ai_c):
     # angle (Gx, GH)
     axGH = math.atan2(Hy-Gy, Hx-Gx)
     if(abs(axGH+leg_ext_axle_angle)>radian_epsilon): # check axGH
-      print("ERR097: Error, axGH {:0.3f} is not equal to -1*leg_ext_axle_angle {:0.3f}".format(axGH, leg_ext_axle_angle))
+      six.print_(("ERR097: Error, axGH {:0.3f} is not equal to -1*leg_ext_axle_angle {:0.3f}".format(axGH, leg_ext_axle_angle)))
       sys.exit(2)
     # angle (Gx, GE)
     axGE = math.atan2(Ey-Gy, Ex-Gx)
@@ -359,12 +360,12 @@ def bell_base_main_hole_outline(ai_c):
     lCB = math.sqrt(lAB**2+lAC**2-2*lAB*lAC*math.cos(aBAC)) # law of cosines in the triangle ABC
     cos_aACB = (lCB**2+lAC**2-lAB**2)/(2*lCB*lAC)
     if(abs(cos_aACB)>1):
-      print("ERR359: Error, cos_aACB {:0.3f} is out of the range -1..1".format(cos_aACB))
+      six.print_(("ERR359: Error, cos_aACB {:0.3f} is out of the range -1..1".format(cos_aACB)))
       sys.exit(2)
     aACB = math.acos(cos_aACB)
     cos_aBCD = lCD/lCB
     if(abs(cos_aBCD)>1):
-      print("ERR364: Error, cos_aBCD {:0.3f} is out of the range -1..1".format(cos_aBCD))
+      six.print_(("ERR364: Error, cos_aBCD {:0.3f} is out of the range -1..1".format(cos_aBCD)))
       sys.exit(2)
     aBCD = math.acos(cos_aBCD)
     aACD = aACB + aBCD
@@ -581,7 +582,7 @@ def bell_external_buttress(ai_c, ai_type):
   elif(ai_type=='side'):
     wall_thickness = ai_c['side_thickness']
   else:
-    print("ERR527: Error, ai_type {:s} doesn't exist".format(ai_type))
+    six.print_(("ERR527: Error, ai_type {:s} doesn't exist".format(ai_type)))
     sys.exit(2)
   ### outline construction
   b_p = {}
diff --git a/cnc25d/box_wood_frame.py b/cnc25d/box_wood_frame.py
index 797ff97..9a2156f 100644
--- a/cnc25d/box_wood_frame.py
+++ b/cnc25d/box_wood_frame.py
@@ -31,7 +31,7 @@
 
 #import importing_freecad
 #importing_freecad.importing_freecad()
-import cnc25d_api
+from . import cnc25d_api
 cnc25d_api.importing_freecad()
 
 #print("FreeCAD.Version:", FreeCAD.Version())
@@ -47,7 +47,7 @@
 #import os, errno
 #import os
 #
-from box_wood_frame_outline import *
+from .box_wood_frame_outline import *
 #
 import Part
 #from FreeCAD import Base
diff --git a/cnc25d/box_wood_frame_ng.py b/cnc25d/box_wood_frame_ng.py
index d0488ac..288e5fc 100644
--- a/cnc25d/box_wood_frame_ng.py
+++ b/cnc25d/box_wood_frame_ng.py
@@ -33,7 +33,8 @@
 
 #import importing_freecad
 #importing_freecad.importing_freecad()
-import cnc25d_api
+from . import cnc25d_api
+import six
 cnc25d_api.importing_freecad()
 
 #print("FreeCAD.Version:", FreeCAD.Version())
@@ -169,9 +170,9 @@ def box_wood_frame(ai_constraints):
   bwfdi = box_wood_frame_dictionary_init()
   bwf_c = bwfdi.copy()
   bwf_c.update(ai_constraints)
-  print("dbg155: bwf_c:", bwf_c)
-  if(len(bwf_c.viewkeys() & bwfdi.viewkeys()) != len(bwf_c.viewkeys() | bwfdi.viewkeys())): # check if the dictionary bwf_c has exactly all the keys compare to box_wood_frame_dictionary_init()
-    print("ERR157: Error, bwf_c has too much entries as {:s} or missing entries as {:s}".format(bwf_c.viewkeys() - bwfdi.viewkeys(), bwfdi.viewkeys() - bwf_c.viewkeys()))
+  six.print_(("dbg155: bwf_c:", bwf_c))
+  if(len(bwf_c & bwfdi) != len(bwf_c | bwfdi)): # check if the dictionary bwf_c has exactly all the keys compare to box_wood_frame_dictionary_init()
+    six.print_(("ERR157: Error, bwf_c has too much entries as {:s} or missing entries as {:s}".format(bwf_c - bwfdi, bwfdi - bwf_c)))
     sys.exit(2)
   #print("dbg164: new box_wood_frame constraints:")
   #for k in bwf_c.viewkeys():
@@ -895,7 +896,7 @@ def plank_generic(nai_plank_name, nai_module_width, nai_cutting_extra):
     elif(nai_plank_name=="slab58_front"):
       r_plank = slab_front(nai_cutting_extra)
     else:
-      print("ERR115: Error, the plank_name {:s} doesn't exist".format(nai_plank_name))
+      six.print_(("ERR115: Error, the plank_name {:s} doesn't exist".format(nai_plank_name)))
       sys.exit(2)
     return(r_plank)
   def place_plank_generic(nai_plank_name, nai_module_width, nai_cutting_extra, ai_flip, ai_orientation, ai_position_x, ai_position_y, ai_position_z):
@@ -1163,7 +1164,7 @@ def plank_serie_place(ai_plank_list, nai_module_width, nai_cutting_extra, ai_ini
       elif(l_orientation=='yx'):
         l_inc_x = 1
       else:
-        print("ERR564: the l_orientation {:s} doesn't exist!".format(l_orientation))
+        six.print_(("ERR564: the l_orientation {:s} doesn't exist!".format(l_orientation)))
         sys.exit(2)
       for li in range(l_plank_desc[lp][3]):
         r_batch.append(place_plank_generic(lp, nai_module_width, nai_cutting_extra, 'i', l_orientation, l_pos_x, l_pos_y, ai_ini_z))
@@ -1477,7 +1478,7 @@ def box_wood_frame_text_report(nai_module_width, nai_args_in_txt):
 
   def fg_file_path(ai_output_dir, ai_file_name):
     r_file_path = "{:s}/{:s}".format(ai_output_dir, ai_file_name)
-    print("Generate the output file: {:s}".format(r_file_path))
+    six.print_(("Generate the output file: {:s}".format(r_file_path)))
     return(r_file_path)
 
   def fg_text_report(ai_file_idx, ai_file_name, ai_file_description, ai_size):
@@ -1593,7 +1594,7 @@ def box_wood_frame_file_generator(nai_output_file_basename, nai_args_in_txt):
         It returns a string that contains the file name list and their description.
     """
     bwf_log = box_wood_frame_text_report(ai_module_width, nai_args_in_txt)
-    print("{:s}".format(bwf_log))
+    six.print_(("{:s}".format(bwf_log)))
     l_text_report = bwf_log
     #print("dbg747: nai_output_file_basename:", nai_output_file_basename)
     if(nai_output_file_basename==""):
@@ -1738,7 +1739,7 @@ def box_wood_frame_file_generator(nai_output_file_basename, nai_args_in_txt):
     #r_bwf = frame_assembly(ai_module_width,ai_cutting_extra,0)
     #Part.show(r_bwf)
   else:
-    print("ERR736: Error, return_type {:s} is unknown".format(bwf_c['return_type']))
+    six.print_(("ERR736: Error, return_type {:s} is unknown".format(bwf_c['return_type'])))
     sys.exit(2)
   #r_bwf.exportStl("bwf_assembly.stl")
   #r_bwf = plank_tobo_diagonal(0)
@@ -1818,7 +1819,7 @@ def box_wood_frame_self_test():
   bwf_parser = box_wood_frame_add_argument(bwf_parser)
   for i in range(len(test_case_switch)):
     l_test_switch = test_case_switch[i][1]
-    print("{:2d} test case: '{:s}'\nwith switch: {:s}".format(i, test_case_switch[i][0], l_test_switch))
+    six.print_(("{:2d} test case: '{:s}'\nwith switch: {:s}".format(i, test_case_switch[i][0], l_test_switch)))
     l_args = l_test_switch.split()
     #print("dbg414: l_args:", l_args)
     st_args = bwf_parser.parse_args(l_args)
diff --git a/cnc25d/box_wood_frame_outline.py b/cnc25d/box_wood_frame_outline.py
index 03ffea2..c785915 100644
--- a/cnc25d/box_wood_frame_outline.py
+++ b/cnc25d/box_wood_frame_outline.py
@@ -30,7 +30,7 @@
 
 #import importing_freecad
 #importing_freecad.importing_freecad()
-import cnc25d_api
+from . import cnc25d_api
 cnc25d_api.importing_freecad()
 
 #print("FreeCAD.Version:", FreeCAD.Version())
diff --git a/cnc25d/cnc25d_api.py b/cnc25d/cnc25d_api.py
index b8c8a76..2cbf3ce 100644
--- a/cnc25d/cnc25d_api.py
+++ b/cnc25d/cnc25d_api.py
@@ -29,16 +29,16 @@
 # import
 ################################################################
 
-import importing_freecad
-import cnc_outline
-import outline_backends
-import positioning
-import export_2d
-import design_output
-import design_help
-import bare_design
-import design_frontend
-import draw_2d_frontend
+from . import importing_freecad
+from . import cnc_outline
+from . import outline_backends
+from . import positioning
+from . import export_2d
+from . import design_output
+from . import design_help
+from . import bare_design
+from . import design_frontend
+from . import draw_2d_frontend
 
 ################################################################
 # api function alias
diff --git a/cnc25d/cnc25d_design.py b/cnc25d/cnc25d_design.py
index 6034ae5..313099b 100644
--- a/cnc25d/cnc25d_design.py
+++ b/cnc25d/cnc25d_design.py
@@ -29,22 +29,22 @@
 # import
 ################################################################
 
-import box_wood_frame
-import gear_profile
-import gearwheel
-import gearring
-import gearbar
-import split_gearwheel
-import epicyclic_gearing
-import axle_lid
-import motor_lid
-import bell
-import bagel
-import bell_bagel_assembly
-import crest
-import cross_cube
-import gimbal
-import low_torque_transmission
+from . import box_wood_frame
+from . import gear_profile
+from . import gearwheel
+from . import gearring
+from . import gearbar
+from . import split_gearwheel
+from . import epicyclic_gearing
+from . import axle_lid
+from . import motor_lid
+from . import bell
+from . import bagel
+from . import bell_bagel_assembly
+from . import crest
+from . import cross_cube
+from . import gimbal
+from . import low_torque_transmission
 
 ################################################################
 # Cnc25d Designs
diff --git a/cnc25d/cnc_outline.py b/cnc25d/cnc_outline.py
index bbb4717..bee53e0 100644
--- a/cnc25d/cnc_outline.py
+++ b/cnc25d/cnc_outline.py
@@ -28,7 +28,7 @@
 # python behavior
 ################################################################
 
-from __future__ import division # to get float division
+ # to get float division
 
 ################################################################
 # header for Python / FreeCAD compatibility
@@ -54,8 +54,9 @@
 import math
 import sys, argparse
 #
-import design_help # just for get_effective_args()
-from small_geometry import *
+from . import design_help # just for get_effective_args()
+from .small_geometry import *
+import six
 
 ################################################################
 # ******** Sub-functions for the API ***********
@@ -69,7 +70,7 @@ def check_outline_format(ai_outline):
   # check if it is a format-B circle
   if(not isinstance(ai_outline, (tuple, list))):
     print("ERR937: Error, ai_outline must be a list or a tuple")
-    print("dbg072: ai_outline:", ai_outline)
+    six.print_(("dbg072: ai_outline:", ai_outline))
     sys.exit(2)
   # check if the outline contains at least on element
   if(len(ai_outline)==0):
@@ -79,8 +80,8 @@ def check_outline_format(ai_outline):
   if(isinstance(ai_outline[0], (tuple, list))): # general outline
     # checks on ai_outline for general outline
     if(len(ai_outline)<2):
-      print("ERR402: Error, the segment list must contain at least 2 elements. Currently, len(ai_outline) = {:d}".format(len(ai_outline)))
-      print("dbg082: ai_outline:", ai_outline)
+      six.print_(("ERR402: Error, the segment list must contain at least 2 elements. Currently, len(ai_outline) = {:d}".format(len(ai_outline))))
+      six.print_(("dbg082: ai_outline:", ai_outline))
       sys.exit(2)
     # check the first point
     len_first_point = len(ai_outline[0])
@@ -89,12 +90,12 @@ def check_outline_format(ai_outline):
     elif(len_first_point==3):
       r_outline_type = 2
     else:
-      print("ERR457: Error, the first point has an unexpected number of items {:d}".format(len_first_point))
+      six.print_(("ERR457: Error, the first point has an unexpected number of items {:d}".format(len_first_point)))
       #print("dbg093: ai_outline:", ai_outline)
       sys.exit(2)
   else: # circle outline
     if(len(ai_outline)!=3):
-      print("ERR758: Error, circle outline must be a list of 3 floats (or int)! Current len: {:d}".format(len(ai_outline)))
+      six.print_(("ERR758: Error, circle outline must be a list of 3 floats (or int)! Current len: {:d}".format(len(ai_outline))))
       sys.exit(2)
     r_outline_type = 0
   return(r_outline_type)
@@ -125,7 +126,7 @@ def reverse_outline(ai_outline):
       elif(len_p==4):
         is_arc = True
       else:
-        print("ERR257: Error, the segment has an unxepected number of items {:d}".format(len_p))
+        six.print_(("ERR257: Error, the segment has an unxepected number of items {:d}".format(len_p)))
         sys.exit(2)
     elif(outline_type==2):
       if(len_p==3):
@@ -133,7 +134,7 @@ def reverse_outline(ai_outline):
       elif(len_p==5):
         is_arc = True
       else:
-        print("ERR258: Error, the segment has an unxepected number of items {:d}".format(len_p))
+        six.print_(("ERR258: Error, the segment has an unxepected number of items {:d}".format(len_p)))
         sys.exit(2)
     #else:
     #  print("ERR557: Error, the outline_type is unexpected {:d}".format(outline_type))
@@ -179,7 +180,7 @@ def reverse_outline(ai_outline):
   if(outline_closed):
     if(outline_type==2):
       if(r_outline[0][2]!=0):
-        print("WARN567: Warning, the last router_bit request of the closed outline is not set to zero: {:0.2f}".format(r_outline[0][2]))
+        six.print_(("WARN567: Warning, the last router_bit request of the closed outline is not set to zero: {:0.2f}".format(r_outline[0][2])))
       r_outline[0]=(r_outline[0][0], r_outline[0][1], r_outline[-1][-1])
       last_segment =  list(r_outline[-1])
       last_segment[-1] = 0
@@ -203,22 +204,22 @@ def smooth_corner_line_line(ai_pre_point, ai_current_point, ai_post_point, ai_ro
   # corner angle
   #if(AG==0):
   if(AG<radian_epsilon):
-    print("ERR406: the length AG is null at point {:s} ({:0.2f}, {:0.2f}, {:0.2f})".format(ai_error_msg_id, AX, AY, ai_router_bit_request))
+    six.print_(("ERR406: the length AG is null at point {:s} ({:0.2f}, {:0.2f}, {:0.2f})".format(ai_error_msg_id, AX, AY, ai_router_bit_request)))
     sys.exit(1)
   #if(AH==0):
   if(AH<radian_epsilon):
-    print("ERR407: the length AH is null at point {:s} ({:0.2f}, {:0.2f}, {:0.2f})".format(ai_error_msg_id, AX, AY, ai_router_bit_request))
+    six.print_(("ERR407: the length AH is null at point {:s} ({:0.2f}, {:0.2f}, {:0.2f})".format(ai_error_msg_id, AX, AY, ai_router_bit_request)))
     sys.exit(1)
   #if(GH==0):
   if(GH<radian_epsilon):
-    print("ERR408: the length GH is null at point {:s} ({:0.2f}, {:0.2f}, {:0.2f})".format(ai_error_msg_id, AX, AY, ai_router_bit_request))
+    six.print_(("ERR408: the length GH is null at point {:s} ({:0.2f}, {:0.2f}, {:0.2f})".format(ai_error_msg_id, AX, AY, ai_router_bit_request)))
     sys.exit(1)
   # law of cosines
   #corner_angle = math.acos((AG**2+AH**2-GH**2)/(2*AH*AG))
   corner_cos = (AG**2+AH**2-GH**2)/(2*AH*AG)
   if(abs(corner_cos)>(1+radian_epsilon)):
-    print("ERR210: Error with math.acos {:0.5f}".format((AG**2+AH**2-GH**2)/(2*AH*AG)))
-    print("dbg211: AG {:0.3f}  AH {:0.3f}  GH {:0.3f}".format(AG, AH, GH))
+    six.print_(("ERR210: Error with math.acos {:0.5f}".format((AG**2+AH**2-GH**2)/(2*AH*AG))))
+    six.print_(("dbg211: AG {:0.3f}  AH {:0.3f}  GH {:0.3f}".format(AG, AH, GH)))
     sys.exit(2)
   elif(corner_cos>=1):
     corner_angle = 0
@@ -228,16 +229,16 @@ def smooth_corner_line_line(ai_pre_point, ai_current_point, ai_post_point, ai_ro
     corner_angle = math.acos(corner_cos)
   #print("dbg206: corner_angle:", corner_angle)
   if(corner_angle>math.pi-radian_epsilon):
-    print("WARN673: Warning, the corner angle {:s} is too flat and won't be smoothed!".format(ai_error_msg_id))
+    six.print_(("WARN673: Warning, the corner angle {:s} is too flat and won't be smoothed!".format(ai_error_msg_id)))
     r_outline = [(AX, AY)]
   elif(corner_angle<radian_epsilon):
-    print("WARN674: Warning, the corner angle {:s} is too acute and won't be smoothed!".format(ai_error_msg_id))
+    six.print_(("WARN674: Warning, the corner angle {:s} is too acute and won't be smoothed!".format(ai_error_msg_id)))
     r_outline = [(AX, AY)]
   else:
     # AE ( = AF = r/(tan(a/2) )
     AE = ai_router_bit_request/math.tan(corner_angle/2)
     if((AG<AE)or(AH<AE)):
-      print("WARN675: Warning, the segments of {:s} are too small to smooth the corner: AG={:0.2f} AH={:0.2f} AE={:0.2f}!".format(ai_error_msg_id, AG, AH, AE))
+      six.print_(("WARN675: Warning, the segments of {:s} are too small to smooth the corner: AG={:0.2f} AH={:0.2f} AE={:0.2f}!".format(ai_error_msg_id, AG, AH, AE)))
       r_outline = [(AX, AY)]
     else:
       # coordiantes of E and F
@@ -311,29 +312,29 @@ def enlarge_corner_line_line(ai_pre_point, ai_current_point, ai_post_point, ai_r
   # corner angle
   #if(AG==0):
   if(AG<radian_epsilon):
-    print("ERR506: the length AG is null at point {:s} ({:0.2f}, {:0.2f}, {:0.2f})".format(ai_error_msg_id, AX, AY, ai_router_bit_request))
+    six.print_(("ERR506: the length AG is null at point {:s} ({:0.2f}, {:0.2f}, {:0.2f})".format(ai_error_msg_id, AX, AY, ai_router_bit_request)))
     sys.exit(1)
   #if(AH==0):
   if(AH<radian_epsilon):
-    print("ERR507: the length AH is null at point {:s} ({:0.2f}, {:0.2f}, {:0.2f})".format(ai_error_msg_id, AX, AY, ai_router_bit_request))
+    six.print_(("ERR507: the length AH is null at point {:s} ({:0.2f}, {:0.2f}, {:0.2f})".format(ai_error_msg_id, AX, AY, ai_router_bit_request)))
     sys.exit(1)
   #if(GH==0):
   if(GH<radian_epsilon):
-    print("ERR508: the length GH is null at point {:s} ({:0.2f}, {:0.2f}, {:0.2f})".format(ai_error_msg_id, AX, AY, ai_router_bit_request))
+    six.print_(("ERR508: the length GH is null at point {:s} ({:0.2f}, {:0.2f}, {:0.2f})".format(ai_error_msg_id, AX, AY, ai_router_bit_request)))
     sys.exit(1)
   corner_angle = math.acos((AG**2+AH**2-GH**2)/(2*AH*AG))
   #print("dbg296: corner_angle:", corner_angle)
   if(corner_angle>math.pi-radian_epsilon):
-    print("WARN573: Warning, the corner angle {:s} is too flat and won't be enlarged!".format(ai_error_msg_id))
+    six.print_(("WARN573: Warning, the corner angle {:s} is too flat and won't be enlarged!".format(ai_error_msg_id)))
     r_outline = [(AX, AY)]
   elif(corner_angle<radian_epsilon):
-    print("WARN574: Warning, the corner angle {:s} is too acute and won't be enlarged!".format(ai_error_msg_id))
+    six.print_(("WARN574: Warning, the corner angle {:s} is too acute and won't be enlarged!".format(ai_error_msg_id)))
     r_outline = [(AX, AY)]
   elif(corner_angle>math.pi/2-radian_epsilon): # enlarge obtuse angle
     # AE ( = AF = 2*r*cos(a/2) )
     AE = 2*ai_router_bit_request*math.cos(corner_angle/2)
     if((AG<AE)or(AH<AE)):
-      print("WARN575: Warning, the segments of {:s} are too small to enlarge the obtuse corner: AG={:0.2f} AH={:0.2f} AE={:0.2f}!".format(ai_error_msg_id, AG, AH, AE))
+      six.print_(("WARN575: Warning, the segments of {:s} are too small to enlarge the obtuse corner: AG={:0.2f} AH={:0.2f} AE={:0.2f}!".format(ai_error_msg_id, AG, AH, AE)))
       r_outline = [(AX, AY)]
     else:
       # coordiantes of E and F
@@ -348,7 +349,7 @@ def enlarge_corner_line_line(ai_pre_point, ai_current_point, ai_post_point, ai_r
     AM = ai_router_bit_request/math.sin(corner_angle/2)
     #print("dbg503: AM:", AM)
     if((AG<AM)or(AH<AM)):
-      print("WARN578: Warning, the segments of {:s} are too small to enlarge the acute corner: AG={:0.2f} AH={:0.2f} AM={:0.2f}!".format(ai_error_msg_id, AG, AH, AM))
+      six.print_(("WARN578: Warning, the segments of {:s} are too small to enlarge the acute corner: AG={:0.2f} AH={:0.2f} AM={:0.2f}!".format(ai_error_msg_id, AG, AH, AM)))
       r_outline = [(AX, AY)]
     else:
       # coordiantes of M and N
@@ -517,16 +518,16 @@ def arc_middle(ai_arc_pt1, ai_arc_pt2, ai_arc_pt3, ai_new_end1, ai_new_end2, ai_
   ID = math.sqrt((DX-IX)**2+(DY-IY)**2)
   IE = math.sqrt((EX-IX)**2+(EY-IY)**2)
   if((abs(IA-arc_radius)>radian_epsilon)or(abs(IB-arc_radius)>radian_epsilon)or(abs(IC-arc_radius)>radian_epsilon)):
-    print("ERR831: Error, in {:s}, I is not equidistant from A,B,C,D,E. IA={:0.2f} IB={:0.2f} IC={:0.2f}".format(ai_error_msg_id, IA, IB, IC))
+    six.print_(("ERR831: Error, in {:s}, I is not equidistant from A,B,C,D,E. IA={:0.2f} IB={:0.2f} IC={:0.2f}".format(ai_error_msg_id, IA, IB, IC)))
     sys.exit(2)
   if((abs(ID-IA)>radian_epsilon)or(abs(IE-IA)>radian_epsilon)):
-    print("ERR832: Error, in {:s}, I is not equidistant from A,B,C,D,E. IA={:0.2f} IB={:0.2f} IC={:0.2f} ID={:0.2f} IE={:0.2f}".format(error_msg_id, IA, IB, IC, ID, IE))
-    print("dbg414: AX {:0.2f}  AY {:0.2f}".format(AX,AY))
-    print("dbg424: BX {:0.2f}  BY {:0.2f}".format(BX,BY))
-    print("dbg434: CX {:0.2f}  CY {:0.2f}".format(CX,CY))
-    print("dbg444: DX {:0.2f}  DY {:0.2f}".format(DX,DY))
-    print("dbg454: EX {:0.2f}  EY {:0.2f}".format(EX,EY))
-    print("dbg464: IX {:0.2f}  IY {:0.2f}".format(IX,IY))
+    six.print_(("ERR832: Error, in {:s}, I is not equidistant from A,B,C,D,E. IA={:0.2f} IB={:0.2f} IC={:0.2f} ID={:0.2f} IE={:0.2f}".format(error_msg_id, IA, IB, IC, ID, IE)))
+    six.print_(("dbg414: AX {:0.2f}  AY {:0.2f}".format(AX,AY)))
+    six.print_(("dbg424: BX {:0.2f}  BY {:0.2f}".format(BX,BY)))
+    six.print_(("dbg434: CX {:0.2f}  CY {:0.2f}".format(CX,CY)))
+    six.print_(("dbg444: DX {:0.2f}  DY {:0.2f}".format(DX,DY)))
+    six.print_(("dbg454: EX {:0.2f}  EY {:0.2f}".format(EX,EY)))
+    six.print_(("dbg464: IX {:0.2f}  IY {:0.2f}".format(IX,IY)))
     sys.exit(2)
   # calculation of the angles d=(Ix, ID) and e=(Ix, IE)
   d = math.atan2(DY-IY, DX-IX)
@@ -540,21 +541,21 @@ def arc_middle(ai_arc_pt1, ai_arc_pt2, ai_arc_pt3, ai_new_end1, ai_new_end2, ai_
     ue = math.fmod(e-u+4*math.pi, 2*math.pi)
   # correction because of calculation imprecision
   if(abs(ud)>2*math.pi-radian_epsilon):
-    print("WARN771: Warning in {:s}, the angle ud is corrected to zero because of presumed calculation imprecision!".format(error_msg_id))
+    six.print_(("WARN771: Warning in {:s}, the angle ud is corrected to zero because of presumed calculation imprecision!".format(error_msg_id)))
     ud = 0
   arc_middle_status = 1
   # check if E and F below to the arc segment A,B,C
   if((abs(uw)<abs(ud)-radian_epsilon)or(abs(uw)<abs(ue)-radian_epsilon)):
-    print("ERR441: Error, in {:s}, E or F are not on the arc segment A,B,C! uw={:0.2f} ud={:0.2f} ue={:0.2f}".format(error_msg_id, uw, ud, ue))
-    print("dbg927: A={:0.2f} {:0.2f}  B={:0.2f} {:0.2f}  C={:0.2f} {:0.2f}  D={:0.2f} {:0.2f}  E={:0.2f} {:0.2f}".format(AX,AY,BX,BY,CX,CY,DX,DY,EX,EY))
+    six.print_(("ERR441: Error, in {:s}, E or F are not on the arc segment A,B,C! uw={:0.2f} ud={:0.2f} ue={:0.2f}".format(error_msg_id, uw, ud, ue)))
+    six.print_(("dbg927: A={:0.2f} {:0.2f}  B={:0.2f} {:0.2f}  C={:0.2f} {:0.2f}  D={:0.2f} {:0.2f}  E={:0.2f} {:0.2f}".format(AX,AY,BX,BY,CX,CY,DX,DY,EX,EY)))
     #sys.exit(2)
     arc_middle_status = 2
   # check the orientation of E,F compare to A,B,C
   if(abs(ud)>abs(ue)):
-    print("dbg539: u {:0.3f}  uw {:0.3f}  w {:0.3f}  d {:0.3f}  e {:0.3f}".format(u, uw, w, d, e))
-    print("dbg540: AX {:0.3f}  AY {:0.3f}  BX {:0.3f}  BY {:0.3f}  CX {:0.3f}  CY {:0.3f}  DX {:0.3f}  DY {:0.3f}  EX {:0.3f}  EY {:0.3f}".format(AX, AY, BX, BY, CX, CY, DX, DY, EX, EY))
-    print("dbg541: ud {:0.3f}  ue {:0.3f}".format(ud, ue))
-    print("ERR442: Error, in {:s}, E,F have not the same orientation as A,B,C! ud={:0.2f} ue={:0.2f}".format(error_msg_id, ud, ue))
+    six.print_(("dbg539: u {:0.3f}  uw {:0.3f}  w {:0.3f}  d {:0.3f}  e {:0.3f}".format(u, uw, w, d, e)))
+    six.print_(("dbg540: AX {:0.3f}  AY {:0.3f}  BX {:0.3f}  BY {:0.3f}  CX {:0.3f}  CY {:0.3f}  DX {:0.3f}  DY {:0.3f}  EX {:0.3f}  EY {:0.3f}".format(AX, AY, BX, BY, CX, CY, DX, DY, EX, EY)))
+    six.print_(("dbg541: ud {:0.3f}  ue {:0.3f}".format(ud, ue)))
+    six.print_(("ERR442: Error, in {:s}, E,F have not the same orientation as A,B,C! ud={:0.2f} ue={:0.2f}".format(error_msg_id, ud, ue)))
     #sys.exit(2)
     arc_middle_status = 2
   # calculation of the angle de=(ID, IE)
@@ -563,7 +564,7 @@ def arc_middle(ai_arc_pt1, ai_arc_pt2, ai_arc_pt3, ai_new_end1, ai_new_end2, ai_
   else:
     de = math.fmod(e-d+4*math.pi, 2*math.pi)
   if(arc_middle_status == 2):
-    print("dbg550: de {:0.3f}  arc_radius {:0.3f}".format(de, arc_radius))
+    six.print_(("dbg550: de {:0.3f}  arc_radius {:0.3f}".format(de, arc_radius)))
   # calculation of the angle f=(Ix, IF)=(Ix, ID)+(ID, IF)
   f = d + de/2
   # calculation of F
@@ -571,7 +572,7 @@ def arc_middle(ai_arc_pt1, ai_arc_pt2, ai_arc_pt3, ai_new_end1, ai_new_end2, ai_
   FY = IY+arc_radius*math.sin(f)
   # dummy F in case of error
   if(arc_middle_status==2):
-    print("ERR221: Error in {:s} during the recalculation of the arc middle point!".format(error_msg_id))
+    six.print_(("ERR221: Error in {:s} during the recalculation of the arc middle point!".format(error_msg_id)))
     sys.exit(2)
     #print("WARN221: Warning in {:s}, creating a dummy arc because of internal error!".format(error_msg_id))
     #lDE = math.sqrt((EX-DX)**2+(EY-DY)**2)
@@ -592,7 +593,7 @@ def general_outline_shift_xy(ai_outline, ai_x_offset, ai_x_coefficient, ai_y_off
   """
   # check the parameters
   if((ai_x_coefficient==0)or(ai_y_coefficient==0)):
-    print("ERR439: Error, a multiplication coefficient is set to zero: {:0.2f}  {:0.2f}".format(ai_x_coefficient, ai_y_coefficient))
+    six.print_(("ERR439: Error, a multiplication coefficient is set to zero: {:0.2f}  {:0.2f}".format(ai_x_coefficient, ai_y_coefficient)))
     sys.exit(2)
   # check if the outline must be reversed
   if((ai_x_coefficient*ai_y_coefficient)<0):
@@ -615,7 +616,7 @@ def general_outline_shift_xy(ai_outline, ai_x_offset, ai_x_coefficient, ai_y_off
       elif(len_p==4):
         is_arc = True
       else:
-        print("ERR237: Error, the segment has an unxepected number of items {:d}".format(len_p))
+        six.print_(("ERR237: Error, the segment has an unxepected number of items {:d}".format(len_p)))
         sys.exit(2)
     elif(outline_type==2):
       if(len_p==3):
@@ -623,10 +624,10 @@ def general_outline_shift_xy(ai_outline, ai_x_offset, ai_x_coefficient, ai_y_off
       elif(len_p==5):
         is_arc = True
       else:
-        print("ERR247: Error, the segment has an unxepected number of items {:d}".format(len_p))
+        six.print_(("ERR247: Error, the segment has an unxepected number of items {:d}".format(len_p)))
         sys.exit(2)
     else:
-      print("ERR257: Error, the outline_type is unexpected {:d}".format(outline_type))
+      six.print_(("ERR257: Error, the outline_type is unexpected {:d}".format(outline_type)))
       sys.exit(2)
     # extract segments
     end_point = []
@@ -673,7 +674,7 @@ def general_outline_rotate(ai_outline, ai_ox, ai_oy, ai_rotation_angle):
       elif(len_p==4):
         is_arc = True
       else:
-        print("ERR237: Error, the segment has an unxepected number of items {:d}".format(len_p))
+        six.print_(("ERR237: Error, the segment has an unxepected number of items {:d}".format(len_p)))
         sys.exit(2)
     elif(outline_type==2):
       if(len_p==3):
@@ -681,10 +682,10 @@ def general_outline_rotate(ai_outline, ai_ox, ai_oy, ai_rotation_angle):
       elif(len_p==5):
         is_arc = True
       else:
-        print("ERR247: Error, the segment has an unxepected number of items {:d}".format(len_p))
+        six.print_(("ERR247: Error, the segment has an unxepected number of items {:d}".format(len_p)))
         sys.exit(2)
     else:
-      print("ERR257: Error, the outline_type is unexpected {:d}".format(outline_type))
+      six.print_(("ERR257: Error, the outline_type is unexpected {:d}".format(outline_type)))
       sys.exit(2)
     # extract segments
     end_point = []
@@ -722,7 +723,7 @@ def outline_shift_xy(ai_outline, ai_x_offset, ai_x_coefficient, ai_y_offset, ai_
   outline_type = check_outline_format(ai_outline)
   if(outline_type==0): # it's a format-B circle
     if(abs(ai_x_coefficient)!=abs(ai_y_coefficient)):
-      print("WARN358: Warning, circle is scaled with different scale along x and y! {:0.2f} {:0.2f}".format(ai_x_coefficient, ai_y_coefficient))
+      six.print_(("WARN358: Warning, circle is scaled with different scale along x and y! {:0.2f} {:0.2f}".format(ai_x_coefficient, ai_y_coefficient)))
     circle_center_x = ai_outline[0]
     circle_center_y = ai_outline[1]
     circle_radius = ai_outline[2]
@@ -834,14 +835,14 @@ def cnc_cut_outline(ai_segment_list, ai_error_msg_id):
   segment_nb = point_nb-1
   # check of the outline size
   if(segment_nb<1):
-    print("ERR202: Error in {:s}, the number of segments must be bigger than 1. Currently: {:d}".format(ai_error_msg_id, segment_nb))
+    six.print_(("ERR202: Error in {:s}, the number of segments must be bigger than 1. Currently: {:d}".format(ai_error_msg_id, segment_nb)))
     sys.exit(2)
   if((segment_nb<2)and(outline_closed)):
-    print("ERR203: Error in {:s}, the number of segments must be bigger than 2 with a closed outline. Currently: {:d}".format(ai_error_msg_id, point_nb))
+    six.print_(("ERR203: Error in {:s}, the number of segments must be bigger than 2 with a closed outline. Currently: {:d}".format(ai_error_msg_id, point_nb)))
     sys.exit(2)
   # check the start point
   if(len(ai_segment_list[0])!=3):
-    print("ERR564: the start point is not defined with three floats. {:d}".format(len(ai_segment_list[0])))
+    six.print_(("ERR564: the start point is not defined with three floats. {:d}".format(len(ai_segment_list[0]))))
     sys.exit(2)
   # extract segment data
   pt_end = []
@@ -857,19 +858,19 @@ def cnc_cut_outline(ai_segment_list, ai_error_msg_id):
       (mid_pt_x, mid_pt_y, end_pt_x, end_pt_y, end_pt_r) = ai_segment_list[pt_idx]
       mid_elem = (mid_pt_x, mid_pt_y)
     else:
-      print("ERR563: Error, the segment is defined with an unexpected number of float {:d}".format(len_segment))
+      six.print_(("ERR563: Error, the segment is defined with an unexpected number of float {:d}".format(len_segment)))
       sys.exit(2)
     pt_end.append((end_pt_x,end_pt_y))
     pt_request.append(end_pt_r)
     pt_mid.append(mid_elem)
   # check router_bit request of first and last point
   if((pt_request[0]!=0)and(not outline_closed)):
-    print("WARN946: Warning, in {:s}, the router_bit request of the start point of the open outline is not zero: {:0.2f}".format(ai_error_msg_id, pt_request[0]))
+    six.print_(("WARN946: Warning, in {:s}, the router_bit request of the start point of the open outline is not zero: {:0.2f}".format(ai_error_msg_id, pt_request[0])))
     pt_request[0]=0
   # if the outline is open, the router_bit request of the last point has no signification
   # if the outline is closed, the router_bit request of the last point is ignore. and the router_bit request of the first point is used
   if(pt_request[-1]!=0):
-    print("WARN947: Warning, in {:s}, the router_bit request of the last point of the outline is not zero: {:0.2f}".format(ai_error_msg_id, pt_request[-1]))
+    six.print_(("WARN947: Warning, in {:s}, the router_bit request of the last point of the outline is not zero: {:0.2f}".format(ai_error_msg_id, pt_request[-1])))
     pt_request[-1]=0
   # build outline
   r_outline = []
@@ -945,13 +946,13 @@ def approximate_curve_tangent(ai_polyline, ai_error_msg_id):
   point_nb = len(ai_polyline)
   # check the outline point number
   if(point_nb<2):
-    print("ERR309: Error in {:s}, the number of points must be bigger than 2. Currently: {:d}".format(ai_error_msg_id, point_nb))
+    six.print_(("ERR309: Error in {:s}, the number of points must be bigger than 2. Currently: {:d}".format(ai_error_msg_id, point_nb)))
     sys.exit(2)
   # check if ai_polyline is valid
   for i in range(len(ai_polyline)):
     point_len = len(ai_polyline[i])
     if(point_len!=2):
-      print("ERR319: Error in {:s}, the point {:d} of ai_polyline must have exactly 2 elements. Currently: {:d}".format(ai_error_msg_id, i, point_len))
+      six.print_(("ERR319: Error in {:s}, the point {:d} of ai_polyline must have exactly 2 elements. Currently: {:d}".format(ai_error_msg_id, i, point_len)))
       sys.exit(2)
   # processing initialization
   r_outline = []
@@ -967,7 +968,7 @@ def approximate_curve_tangent(ai_polyline, ai_error_msg_id):
     tangent_inclination2 = math.atan2((post2_point[1]-pre_point[1])/segment_length, (post2_point[0]-pre_point[0])/segment_length)
     tangent_inclination2_diff = math.fmod(tangent_inclination2-tangent_inclination+5*math.pi, 2*math.pi) - math.pi
     if(abs(tangent_inclination2_diff)>math.pi/3):
-      print("ERR315: Error in {:s}, the tangent_inclination is changing too fast for a curve approximation. tangent_inclination: {:0.2f}  tangent_inclination2: {:0.2f}".format(ai_error_msg_id, tangent_inclination, tangent_inclination2))
+      six.print_(("ERR315: Error in {:s}, the tangent_inclination is changing too fast for a curve approximation. tangent_inclination: {:0.2f}  tangent_inclination2: {:0.2f}".format(ai_error_msg_id, tangent_inclination, tangent_inclination2)))
       sys.exit(2)
     tangent_inclination = math.fmod(tangent_inclination-1*tangent_inclination2_diff/2 + 5*math.pi, 2*math.pi) - math.pi
   r_outline.append((ai_polyline[0][0], ai_polyline[0][1], tangent_inclination)) # first-point
@@ -993,7 +994,7 @@ def approximate_curve_tangent(ai_polyline, ai_error_msg_id):
     tangent_inclination2 = math.atan2((post_point[1]-pre2_point[1])/segment_length, (post_point[0]-pre2_point[0])/segment_length)
     tangent_inclination2_diff = math.fmod(tangent_inclination2-tangent_inclination+5*math.pi, 2*math.pi) - math.pi
     if(abs(tangent_inclination2_diff)>math.pi/3):
-      print("ERR315: Error in {:s}, the tangent_inclination is changing too fast for a curve approximation. tangent_inclination: {:0.2f}  tangent_inclination2: {:0.2f}".format(ai_error_msg_id, tangent_inclination, tangent_inclination2))
+      six.print_(("ERR315: Error in {:s}, the tangent_inclination is changing too fast for a curve approximation. tangent_inclination: {:0.2f}  tangent_inclination2: {:0.2f}".format(ai_error_msg_id, tangent_inclination, tangent_inclination2)))
       sys.exit(2)
     tangent_inclination = math.fmod(tangent_inclination-1*tangent_inclination2_diff/2 + 5*math.pi, 2*math.pi) - math.pi
   r_outline.append((ai_polyline[-1][0], ai_polyline[-1][1], tangent_inclination)) # end-point
@@ -1045,13 +1046,13 @@ def ideal_outline(ai_outline, ai_error_msg_id):
       # check the segment length
       segment_len = len(i_segment)
       if((segment_len!=3)and(segment_len!=5)):
-        print("ERR868: Error in {:s}.{:d}, len(segment_len) is not 3 or 5!".format(ai_error_msg_id, i))
+        six.print_(("ERR868: Error in {:s}.{:d}, len(segment_len) is not 3 or 5!".format(ai_error_msg_id, i)))
         sys.exit(2)
       i += 1
       # construct the ideal outline
       r_outline.append(i_segment[:-1]) # remove the third or the fifth element
   else: # format-B circle or format-B general outline
-    print("WARN441: Warning in {:s}, nothing to do, the outline is already in format-B".format(ai_error_msg_id))
+    six.print_(("WARN441: Warning in {:s}, nothing to do, the outline is already in format-B".format(ai_error_msg_id)))
     r_outline = ai_outline
   return(r_outline)
 
@@ -1064,14 +1065,14 @@ def ideal_outline(ai_outline, ai_error_msg_id):
 # addition import for the tests
 ################################################################
 
-import importing_freecad 
+from . import importing_freecad 
 importing_freecad.importing_freecad()
 #
 import Part
 from FreeCAD import Base 
 #
-import Tkinter
-import outline_backends
+from six.moves import tkinter
+from . import outline_backends
 #
 #import timeit
 #import cProfile
@@ -1117,7 +1118,7 @@ def make_H_shape(ai_origin_x, ai_origin_y, ai_router_bit_r, ai_height, ai_output
   Part.show(myh_solid) # works only with FreeCAD GUI, ignore otherwise
   if(ai_output_file!=''):
     myh_solid.exportStl(ai_output_file)
-    print("output stl file: %s"%(ai_output_file))
+    six.print_(("output stl file: %s"%(ai_output_file)))
 
 def make_X_shape(ai_origin_x, ai_origin_y, ai_router_bit_r, ai_height, ai_output_file):
   """ design a X-shape to test not 90 degree angles with the function cnc_cut_outline()
@@ -1154,7 +1155,7 @@ def make_X_shape(ai_origin_x, ai_origin_y, ai_router_bit_r, ai_height, ai_output
   Part.show(myx_solid) # works only with FreeCAD GUI, ignore otherwise
   if(ai_output_file!=''):
     myx_solid.exportStl(ai_output_file)
-    print("output stl file: %s"%(ai_output_file))
+    six.print_(("output stl file: %s"%(ai_output_file)))
 
 def make_M_shape(ai_origin_x, ai_origin_y, ai_router_bit_r, ai_height, ai_output_file):
   """ design a M-shape to test the three outline_shift_x, _y and _xy functions
@@ -1201,7 +1202,7 @@ def make_M_shape(ai_origin_x, ai_origin_y, ai_router_bit_r, ai_height, ai_output
   Part.show(myx_solid) # works only with FreeCAD GUI, ignore otherwise
   if(ai_output_file!=''):
     myx_solid.exportStl(ai_output_file)
-    print("output stl file: %s"%(ai_output_file))
+    six.print_(("output stl file: %s"%(ai_output_file)))
 
 def cnc_cut_outline_test1():
   """ First test to check the cnc_cut_outline API
@@ -1209,7 +1210,7 @@ def cnc_cut_outline_test1():
   ### check the cnc_cut_outline function with several router_bit diameter
   y_offset = 0
   for ir in [2.0, 1.5, 1.0, 0, -1.0, -2.0]:
-    print("dbg603: test with router_bit radius (ir):", ir)
+    six.print_(("dbg603: test with router_bit radius (ir):", ir))
     make_H_shape( 0,  y_offset, ir, 2, "self_test_cnc_cut_outline_H_r%0.2f.stl"%ir)
     make_X_shape(50,  y_offset, ir, 2, "self_test_cnc_cut_outline_X_r%0.2f.stl"%ir)
     make_M_shape(150,  y_offset, ir, 2, "self_test_cnc_cut_outline_M_r%0.2f.stl"%ir)
@@ -1238,7 +1239,7 @@ def cnc_cut_outline_test3(ai_sw_router_bit_radius):
   """ Third test to check the cnc_cut_outline API
       It displays the shapes with Tkinter
   """
-  print("Run test_3  with ai_sw_router_bit_radius={:0.2f} ...".format(ai_sw_router_bit_radius))
+  six.print_(("Run test_3  with ai_sw_router_bit_radius={:0.2f} ...".format(ai_sw_router_bit_radius)))
   def outline_a(ai_router_bit_radius):
     corner_a=[
       [0,20, ai_router_bit_radius],
@@ -1332,7 +1333,7 @@ def outline_a(ai_router_bit_radius):
   outline_e2 = smooth_outline_b_curve(first_curve, radian_precision, ai_sw_router_bit_radius, 'cnc_cut_outline_test3_e2')
 
   # display with Tkinter
-  tk_root = Tkinter.Tk()
+  tk_root = tkinter.Tk()
   my_canvas = outline_backends.Two_Canvas(tk_root)
   # callback function for display_backend
   def sub_canvas_graphics(ai_rotation_direction, ai_angle_position):
@@ -1374,7 +1375,7 @@ def measure_the_execution_time_of_the_sub_canvas_graphics_function(ai_repeat_nb)
     for i in range(ai_repeat_nb):
       tmp = sub_canvas_graphics(1, i*math.pi/200)
     time_stop = time.clock()
-    print("dbg507: time sub_canvas_graphics:", time_stop-time_start)
+    six.print_(("dbg507: time sub_canvas_graphics:", time_stop-time_start))
   #measure_the_execution_time_of_the_sub_canvas_graphics_function(100)
   # end of measurement
   my_canvas.add_canvas_graphic_function(sub_canvas_graphics)
@@ -1387,7 +1388,7 @@ def cnc_cut_outline_test4(ai_sw_router_bit_radius):
   """ Fourth test suggests small shapes, useful to develop the cnc_cut_outline API
       It displays the shapes with Tkinter
   """
-  print("Run test_4  with ai_sw_router_bit_radius={:0.2f} ...".format(ai_sw_router_bit_radius))
+  six.print_(("Run test_4  with ai_sw_router_bit_radius={:0.2f} ...".format(ai_sw_router_bit_radius)))
   def outline_t4_a(ai_router_bit_radius):
     three_arcs=[
       [0,20, 0],
@@ -1435,7 +1436,7 @@ def outline_t4_a(ai_router_bit_radius):
   #  print("dbg332: outline_a2 i_segment:", len(i_segment), i_segment)
 
   # display with Tkinter
-  tk_root = Tkinter.Tk()
+  tk_root = tkinter.Tk()
   my_canvas = outline_backends.Two_Canvas(tk_root)
   # callback function for display_backend
   def sub_canvas_graphics_t4(ai_rotation_direction, ai_angle_position):
@@ -1458,7 +1459,7 @@ def cnc_cut_outline_test5(ai_sw_router_bit_radius):
       It displays the shapes with Tkinter
   """
   radian_precision = math.pi/100
-  print("Run test_5  with ai_sw_router_bit_radius={:0.2f} ...".format(ai_sw_router_bit_radius))
+  six.print_(("Run test_5  with ai_sw_router_bit_radius={:0.2f} ...".format(ai_sw_router_bit_radius)))
   first_curve=[
     [20,0],
     [22,10],
@@ -1476,7 +1477,7 @@ def cnc_cut_outline_test5(ai_sw_router_bit_radius):
   #  print("dbg332: outline_a2 i_segment:", len(i_segment), i_segment)
 
   # display with Tkinter
-  tk_root = Tkinter.Tk()
+  tk_root = tkinter.Tk()
   my_canvas = outline_backends.Two_Canvas(tk_root)
   # callback function for display_backend
   def sub_canvas_graphics_t5(ai_rotation_direction, ai_angle_position):
diff --git a/cnc25d/crest.py b/cnc25d/crest.py
index 9426b3f..17faf8a 100644
--- a/cnc25d/crest.py
+++ b/cnc25d/crest.py
@@ -29,7 +29,8 @@
 # header for Python / FreeCAD compatibility
 ################################################################
 
-import cnc25d_api
+from . import cnc25d_api
+import six
 cnc25d_api.importing_freecad()
 
 #print("FreeCAD.Version:", FreeCAD.Version())
@@ -52,8 +53,8 @@
 #import svgwrite
 #from dxfwrite import DXFEngine
 # cnc25d
-import cross_cube_sub
-import gear_profile
+from . import cross_cube_sub
+from . import gear_profile
 
 
 ################################################################
@@ -150,11 +151,11 @@ def crest_constraint_check(c):
   ### outline
   #gear_module
   if(c['gear_module']<radian_epsilon):
-    print("ERR194: Error, gear_module {:0.3f} must be strictly positive".format(c['gear_module']))
+    six.print_(("ERR194: Error, gear_module {:0.3f} must be strictly positive".format(c['gear_module'])))
     sys.exit(2)
   #virtual_tooth_nb
   if((c['virtual_tooth_nb']-3)*c['gear_module']<0.9*c['cube_width']):
-    print("ERR198: Error, virtual_tooth_nb {:d} is too small compare to gear_module {:0.3f} and cube_width {:0.3f}".format(c['virtual_tooth_nb'], c['gear_module'], c['cube_width']))
+    six.print_(("ERR198: Error, virtual_tooth_nb {:d} is too small compare to gear_module {:0.3f} and cube_width {:0.3f}".format(c['virtual_tooth_nb'], c['gear_module'], c['cube_width'])))
     sys.exit(2)
   gear_hollow_radius = (c['virtual_tooth_nb']-2)*c['gear_module']/2.0
   minimal_gear_portion_angle = 2*math.asin(c['cube_width']/(2.0*gear_hollow_radius))
@@ -162,45 +163,45 @@ def crest_constraint_check(c):
   #portion_tooth_nb
   gear_portion_angle = 2*math.pi*(c['portion_tooth_nb']+1)/c['virtual_tooth_nb']
   if(gear_portion_angle<minimal_gear_portion_angle):
-    print("ERR205: Error, portion_tooth_nb {:d} is too small compare to gear_module {:0.3f}, virtual_tooth_nb {:d} and cube_width {:0.3f}".format(c['portion_tooth_nb'], c['gear_module'], c['virtual_tooth_nb'], c['cube_width']))
+    six.print_(("ERR205: Error, portion_tooth_nb {:d} is too small compare to gear_module {:0.3f}, virtual_tooth_nb {:d} and cube_width {:0.3f}".format(c['portion_tooth_nb'], c['gear_module'], c['virtual_tooth_nb'], c['cube_width'])))
     sys.exit(2)
   if(gear_portion_angle>maximal_gear_portion_angle):
-    print("ERR208: Error, portion_tooth_nb {:d} is too big compare to gear_module {:0.3f}, virtual_tooth_nb {:d}, free_mounting_width {:0.3f} and cube_width {:0.3f}".format(c['portion_tooth_nb'], c['gear_module'], c['virtual_tooth_nb'], c['free_mounting_width'], c['cube_width']))
+    six.print_(("ERR208: Error, portion_tooth_nb {:d} is too big compare to gear_module {:0.3f}, virtual_tooth_nb {:d}, free_mounting_width {:0.3f} and cube_width {:0.3f}".format(c['portion_tooth_nb'], c['gear_module'], c['virtual_tooth_nb'], c['free_mounting_width'], c['cube_width'])))
     sys.exit(2)
   #free_mounting_width
   if(c['free_mounting_width']<max(c['face_B1_thickness'], c['face_B2_thickness'])+c['crest_cnc_router_bit_radius']):
-    print("ERR212: Error, free_mounting_width {:0.3f} is too small compare to face_B1_thickness {:0.3f}, face_B2_thickness {:0.3f} and crest_cnc_router_bit_radius {:0.3f}".format(c['free_mounting_width'], c['face_B1_thickness'], c['face_B2_thickness'], c['crest_cnc_router_bit_radius']))
+    six.print_(("ERR212: Error, free_mounting_width {:0.3f} is too small compare to face_B1_thickness {:0.3f}, face_B2_thickness {:0.3f} and crest_cnc_router_bit_radius {:0.3f}".format(c['free_mounting_width'], c['face_B1_thickness'], c['face_B2_thickness'], c['crest_cnc_router_bit_radius'])))
     sys.exit(2)
   if(c['free_mounting_width']+c['cube_width']/2.0>gear_hollow_radius):
-    print("ERR215: Error, free_mounting_width {:0.3f} is too big compare to cube_width {:0.3f} and gear_hollow_radius {:0.3f}".format(c['free_mounting_width'], c['cube_width'], gear_hollow_radius))
+    six.print_(("ERR215: Error, free_mounting_width {:0.3f} is too big compare to cube_width {:0.3f} and gear_hollow_radius {:0.3f}".format(c['free_mounting_width'], c['cube_width'], gear_hollow_radius)))
     sys.exit(2)
   ### crest_hollow
   #crest_hollow_leg_nb # possible values: 1(filled), 2(end-legs only), 3, 4 ...
   if(c['crest_hollow_leg_nb']<1):
-    print("ERR220: Error, crest_hollow_leg_nb {:d} must be bigger or equal to 1".format(c['crest_hollow_leg_nb']))
+    six.print_(("ERR220: Error, crest_hollow_leg_nb {:d} must be bigger or equal to 1".format(c['crest_hollow_leg_nb'])))
     sys.exit(2)
   #end_leg_width
   if(c['end_leg_width']<radian_epsilon):
-    print("ERR224: Error, end_leg_width {:0.3f} must be strictly positive".format(c['end_leg_width']))
+    six.print_(("ERR224: Error, end_leg_width {:0.3f} must be strictly positive".format(c['end_leg_width'])))
     sys.exit(2)
   if(c['end_leg_width']>0.7*2*math.pi*gear_hollow_radius*gear_portion_angle/c['crest_hollow_leg_nb']):
-    print("ERR228: Error, end_leg_width {:0.3f} is too big compare to gear_hollow_radius {:0.3f}, gear_portion_angle {:0.3f} and crest_hollow_leg_nb {:d}".format(c['end_leg_width'], gear_hollow_radius, gear_portion_angle, c['crest_hollow_leg_nb']))
+    six.print_(("ERR228: Error, end_leg_width {:0.3f} is too big compare to gear_hollow_radius {:0.3f}, gear_portion_angle {:0.3f} and crest_hollow_leg_nb {:d}".format(c['end_leg_width'], gear_hollow_radius, gear_portion_angle, c['crest_hollow_leg_nb'])))
     sys.exit(2)
   #middle_leg_width
   if(c['middle_leg_width']==0):
     c['middle_leg_width'] = c['end_leg_width']
   if(c['middle_leg_width']<radian_epsilon):
-    print("ERR232: Error, middle_leg_width {:0.3f} must be strictly positive".format(c['middle_leg_width']))
+    six.print_(("ERR232: Error, middle_leg_width {:0.3f} must be strictly positive".format(c['middle_leg_width'])))
     sys.exit(2)
   if(c['middle_leg_width']>0.7*2*math.pi*gear_hollow_radius*gear_portion_angle/c['crest_hollow_leg_nb']):
-    print("ERR235: Error, middle_leg_width {:0.3f} is too big compare to gear_hollow_radius {:0.3f}, gear_portion_angle {:0.3f} and crest_hollow_leg_nb {:d}".format(c['middle_leg_width'], gear_hollow_radius, gear_portion_angle, c['crest_hollow_leg_nb']))
+    six.print_(("ERR235: Error, middle_leg_width {:0.3f} is too big compare to gear_hollow_radius {:0.3f}, gear_portion_angle {:0.3f} and crest_hollow_leg_nb {:d}".format(c['middle_leg_width'], gear_hollow_radius, gear_portion_angle, c['crest_hollow_leg_nb'])))
     sys.exit(2)
   #crest_hollow_external_diameter
   c['crest_hollow_external_radius'] = c['crest_hollow_external_diameter']/2.0
   if(c['crest_hollow_external_radius']==0):
     c['crest_hollow_external_radius'] = gear_hollow_radius - 1.5*c['gear_module']
   if(c['crest_hollow_external_radius']>gear_hollow_radius):
-    print("ERR244: Error, crest_hollow_external_radius {:0.3f} is too big compare to gear_hollow_radius {:0.3f}".format(c['crest_hollow_external_radius'], gear_hollow_radius))
+    six.print_(("ERR244: Error, crest_hollow_external_radius {:0.3f} is too big compare to gear_hollow_radius {:0.3f}".format(c['crest_hollow_external_radius'], gear_hollow_radius)))
     sys.exit(2)
   #crest_hollow_internal_diameter
   c['crest_hollow_internal_radius'] = c['crest_hollow_internal_diameter']/2.0
@@ -208,16 +209,16 @@ def crest_constraint_check(c):
   if(c['crest_hollow_internal_radius']==0):
     c['crest_hollow_internal_radius'] = minimal_crest_hollow_internal_radius
   if(c['crest_hollow_internal_radius']<minimal_crest_hollow_internal_radius):
-    print("ERR252: Error, crest_hollow_internal_radius {:0.3f} must be bigger than minimal_crest_hollow_internal_radius {:0.3f}".format(c['crest_hollow_internal_radius'], minimal_crest_hollow_internal_radius))
+    six.print_(("ERR252: Error, crest_hollow_internal_radius {:0.3f} must be bigger than minimal_crest_hollow_internal_radius {:0.3f}".format(c['crest_hollow_internal_radius'], minimal_crest_hollow_internal_radius)))
     sys.exit(2)
   #floor_width
   if(c['floor_width']==0):
     c['floor_width'] = c['end_leg_width']
   if(c['floor_width']<radian_epsilon):
-    print("ERR257: Error, floor_width {:0.3f} must be strictly positive".format(c['floor_width']))
+    six.print_(("ERR257: Error, floor_width {:0.3f} must be strictly positive".format(c['floor_width'])))
     sys.exit(2)
   if(c['floor_width']+c['top_thickness']+c['height_margin']+c['axle_diameter']/2.0>gear_hollow_radius):
-    print("ERR260: Error, floor_width {:0.3f} is too big compare to top_thickness {:0.3f}, height_margin {:0.3f}, axle_diameter {:0.3f} and gear_hollow_radius {:0.3f}".format(c['floor_width'], c['top_thickness'], c['height_margin'], c['axle_diameter'], gear_hollow_radius))
+    six.print_(("ERR260: Error, floor_width {:0.3f} is too big compare to top_thickness {:0.3f}, height_margin {:0.3f}, axle_diameter {:0.3f} and gear_hollow_radius {:0.3f}".format(c['floor_width'], c['top_thickness'], c['height_margin'], c['axle_diameter'], gear_hollow_radius)))
     sys.exit(2)
   #crest_hollow_smoothing_radius
   max_leg_width = max(c['end_leg_width'], c['middle_leg_width'])
@@ -225,35 +226,35 @@ def crest_constraint_check(c):
   if(c['crest_hollow_smoothing_radius']==0):
     c['crest_hollow_smoothing_radius'] = min(0.5*maximal_crest_hollow_smoothing_radius, 0.2*abs(c['crest_hollow_external_radius']-c['crest_hollow_internal_radius']))
   if(c['crest_hollow_smoothing_radius']<c['crest_cnc_router_bit_radius']):
-    print("ERR267: Error, crest_hollow_smoothing_radius {:0.3f} must be bigger than crest_cnc_router_bit_radius {:0.3f}".format(c['crest_hollow_smoothing_radius'], c['crest_cnc_router_bit_radius']))
+    six.print_(("ERR267: Error, crest_hollow_smoothing_radius {:0.3f} must be bigger than crest_cnc_router_bit_radius {:0.3f}".format(c['crest_hollow_smoothing_radius'], c['crest_cnc_router_bit_radius'])))
     sys.exit(2)
   if(c['crest_hollow_smoothing_radius']>maximal_crest_hollow_smoothing_radius):
-    print("ERR270: Error, crest_hollow_smoothing_radius {:0.3f} must be smaller than maximal_crest_hollow_smoothing_radius {:0.3f}".format(c['crest_hollow_smoothing_radius'], maximal_crest_hollow_smoothing_radius))
+    six.print_(("ERR270: Error, crest_hollow_smoothing_radius {:0.3f} must be smaller than maximal_crest_hollow_smoothing_radius {:0.3f}".format(c['crest_hollow_smoothing_radius'], maximal_crest_hollow_smoothing_radius)))
     sys.exit(2)
   if(c['crest_hollow_external_radius']<c['crest_hollow_internal_radius']+2.5*c['crest_hollow_smoothing_radius']):
-    print("ERR265: Error, crest_hollow_external_radius {:0.3f} is too small compare to crest_hollow_internal_radius {:0.3f} and crest_hollow_smoothing_radius {:0.3f}".format(c['crest_hollow_external_radius'], c['crest_hollow_internal_radius'], c['crest_hollow_smoothing_radius']))
+    six.print_(("ERR265: Error, crest_hollow_external_radius {:0.3f} is too small compare to crest_hollow_internal_radius {:0.3f} and crest_hollow_smoothing_radius {:0.3f}".format(c['crest_hollow_external_radius'], c['crest_hollow_internal_radius'], c['crest_hollow_smoothing_radius'])))
     sys.exit(2)
   ### gear_holes
   #fastening_hole_diameter
   c['fastening_hole_radius'] = c['fastening_hole_diameter']/2.0
   #fastening_hole_position
   if(c['crest_hollow_external_radius'] + c['fastening_hole_position'] + c['fastening_hole_radius']>gear_hollow_radius):
-    print("ERR282: Error, fastening_hole_position {:0.3f} or fastening_hole_radius {:0.3f} are too big compare to crest_hollow_external_radius {:0.3f} and gear_hollow_radius {:0.3f}".format(c['fastening_hole_position'], c['fastening_hole_radius'], c['crest_hollow_external_radius'], gear_hollow_radius))
+    six.print_(("ERR282: Error, fastening_hole_position {:0.3f} or fastening_hole_radius {:0.3f} are too big compare to crest_hollow_external_radius {:0.3f} and gear_hollow_radius {:0.3f}".format(c['fastening_hole_position'], c['fastening_hole_radius'], c['crest_hollow_external_radius'], gear_hollow_radius)))
     sys.exit(2)
   #centring_hole_diameter
   c['centring_hole_radius'] = c['centring_hole_diameter']/2.0
   #centring_hole_distance
   if(c['centring_hole_distance']<2.1*c['centring_hole_radius']):
-    print("ERR288: Error, centring_hole_distance {:0.3f} is too small compare to centring_hole_radius {:0.3f}".format(c['centring_hole_distance'], c['centring_hole_radius']))
+    six.print_(("ERR288: Error, centring_hole_distance {:0.3f} is too small compare to centring_hole_radius {:0.3f}".format(c['centring_hole_distance'], c['centring_hole_radius'])))
     sys.exit(2)
   #centring_hole_position
   if(c['crest_hollow_external_radius'] + c['centring_hole_position'] + c['centring_hole_radius']>gear_hollow_radius):
-    print("ERR292: Error, centring_hole_position {:0.3f} or centring_hole_radius {:0.3f} are too big compare to crest_hollow_external_radius {:0.3f} and gear_hollow_radius {:0.3f}".format(c['centring_hole_position'], c['centring_hole_radius'], c['crest_hollow_external_radius'], gear_hollow_radius))
+    six.print_(("ERR292: Error, centring_hole_position {:0.3f} or centring_hole_radius {:0.3f} are too big compare to crest_hollow_external_radius {:0.3f} and gear_hollow_radius {:0.3f}".format(c['centring_hole_position'], c['centring_hole_radius'], c['crest_hollow_external_radius'], gear_hollow_radius)))
     sys.exit(2)
   ## part thickness
   #crest_thickness
   if(c['crest_thickness']<radian_epsilon):
-    print("ERR297: Error, crest_thickness {:0.3f} must be strictly positive".format(c['crest_thickness']))
+    six.print_(("ERR297: Error, crest_thickness {:0.3f} must be strictly positive".format(c['crest_thickness'])))
     sys.exit(2)
   ### manufacturing
   #crest_cnc_router_bit_radius
@@ -290,7 +291,7 @@ def crest_2d_construction(c):
   gear_profile_B = i_gear_profile.get_A_figure('first_gear')[0]
   # gear_profile check
   if(abs(gear_profile_B[0][1]-gear_profile_B[-1][-1])>radian_epsilon):
-    print("ERR335: Error, extremities of the gear_profile_B have different y-coordiante {:0.3f} {:0.3f}".format(gear_profile_B[0][1], gear_profile_B[-1][-1]))
+    six.print_(("ERR335: Error, extremities of the gear_profile_B have different y-coordiante {:0.3f} {:0.3f}".format(gear_profile_B[0][1], gear_profile_B[-1][-1])))
     sys.exit(2)
   # gear_profile extremity angle
   crest_gear_angle = math.atan2(gear_profile_B[0][1]-cy, gear_profile_B[0][0]-cx)
@@ -370,7 +371,7 @@ def crest_2d_construction(c):
       ia1 = in1_angles[i]
       ia2 = in2_angles[i]
       if((ea2-ea1)<2.1*smoothing_ex_half_angle):
-        print("ERR419: Error, crest_hollow_smoothing_radius {:0.3f} or crest_hollow_leg_nb {:d} are too big".format(chsr, chln))
+        six.print_(("ERR419: Error, crest_hollow_smoothing_radius {:0.3f} or crest_hollow_leg_nb {:d} are too big".format(chsr, chln)))
         sys.exit(2)
       hollow = []
       hollow.append((cx+cher*math.cos(ea1), cy+cher*math.sin(ea1), chsr))
diff --git a/cnc25d/cross_cube.py b/cnc25d/cross_cube.py
index c945a19..41b4109 100644
--- a/cnc25d/cross_cube.py
+++ b/cnc25d/cross_cube.py
@@ -29,7 +29,7 @@
 # header for Python / FreeCAD compatibility
 ################################################################
 
-import cnc25d_api
+from . import cnc25d_api
 cnc25d_api.importing_freecad()
 
 #print("FreeCAD.Version:", FreeCAD.Version())
@@ -52,8 +52,8 @@
 #import svgwrite
 #from dxfwrite import DXFEngine
 # cnc25d
-import cross_cube_sub
-import crest
+from . import cross_cube_sub
+from . import crest
 
 ################################################################
 # cross_cube constraint_constructor
diff --git a/cnc25d/cross_cube_sub.py b/cnc25d/cross_cube_sub.py
index 485448f..d7c3793 100644
--- a/cnc25d/cross_cube_sub.py
+++ b/cnc25d/cross_cube_sub.py
@@ -30,7 +30,8 @@
 # header for Python / FreeCAD compatibility
 ################################################################
 
-import cnc25d_api
+from . import cnc25d_api
+import six
 cnc25d_api.importing_freecad()
 
 #print("FreeCAD.Version:", FreeCAD.Version())
@@ -167,30 +168,30 @@ def cross_cube_sub_face_check(ai_constraints):
   # axle_diameter
   cc_c['axle_radius'] = cc_c['axle_diameter']/2.0
   if(cc_c['axle_radius']<radian_epsilon):
-    print("ERR220: Error, axle_radius {:0.3f} must be strictly positive".format(cc_c['axle_radius']))
+    six.print_(("ERR220: Error, axle_radius {:0.3f} must be strictly positive".format(cc_c['axle_radius'])))
     sys.exit(2)
   # inter_axle_length
   if(cc_c['inter_axle_length']<0):
-    print("ERR224: Error, inter_axle_length {:0.3f} must be positive".format(cc_c['inter_axle_length']))
+    six.print_(("ERR224: Error, inter_axle_length {:0.3f} must be positive".format(cc_c['inter_axle_length'])))
     sys.exit(2)
   if(cc_c['inter_axle_length']<2*cc_c['axle_radius']):
-    print("WARN227: Warning, inter_axle_length {:0.3f} is smaller than the axle_diameter {:0.3f}".format(cc_c['inter_axle_length'], 2*cc_c['axle_radius']))
+    six.print_(("WARN227: Warning, inter_axle_length {:0.3f} is smaller than the axle_diameter {:0.3f}".format(cc_c['inter_axle_length'], 2*cc_c['axle_radius'])))
   # height_margin
   if(cc_c['height_margin']<radian_epsilon):
-    print("ERR229: Error, height_margin {:0.3f} must be strictly positive".format(cc_c['height_margin']))
+    six.print_(("ERR229: Error, height_margin {:0.3f} must be strictly positive".format(cc_c['height_margin'])))
     sys.exit(2)
   # top_thickness
   if(cc_c['top_thickness']<2*cc_c['cross_cube_cnc_router_bit_radius']):
-    print("ERR232: Error, top_thickness {:0.3f} is too small compare to cross_cube_cnc_router_bit_radius {:0.3f}".format(cc_c['top_thickness'], cc_c['cross_cube_cnc_router_bit_radius']))
+    six.print_(("ERR232: Error, top_thickness {:0.3f} is too small compare to cross_cube_cnc_router_bit_radius {:0.3f}".format(cc_c['top_thickness'], cc_c['cross_cube_cnc_router_bit_radius'])))
     sys.exit(2)
   if(cc_c['top_thickness']>(cc_c['inter_axle_length']+2*cc_c['axle_radius']+2*cc_c['height_margin'])/4.0):
-    print("ERR235: top_thickness {:0.3f} is too small compare to inter_axle_length {:0.3}, axle_radius {:0.3}, height_margin {:0.3}".format(cc_c['top_thickness'], cc_c['inter_axle_length'], cc_c['axle_radius'], cc_c['height_margin']))
+    six.print_(("ERR235: top_thickness {:0.3f} is too small compare to inter_axle_length {:0.3}, axle_radius {:0.3}, height_margin {:0.3}".format(cc_c['top_thickness'], cc_c['inter_axle_length'], cc_c['axle_radius'], cc_c['height_margin'])))
     sys.exit(2)
   cc_c['cube_height'] = cc_c['inter_axle_length'] + 2*cc_c['axle_radius'] + 2*cc_c['height_margin'] + 2*cc_c['top_thickness']
   ## width
   # cube_width
   if(cc_c['cube_width']<3*cc_c['axle_radius']):
-    print("ERR239: Error, cube_width {:0.3f} is too small compare to axle_radius {:0.3f}".format(cc_c['cube_width'], cc_c['axle_radius']))
+    six.print_(("ERR239: Error, cube_width {:0.3f} is too small compare to axle_radius {:0.3f}".format(cc_c['cube_width'], cc_c['axle_radius'])))
     sys.exit(2)
   # face_A1_thickness, face_A2_thickness, face_B1_thickness, face_B2_thickness
   face_thickness = [cc_c['face_A1_thickness'], cc_c['face_A2_thickness'], cc_c['face_B1_thickness'], cc_c['face_B2_thickness']]
@@ -200,13 +201,13 @@ def cross_cube_sub_face_check(ai_constraints):
     if(face_thickness[i]==0):
       face_thickness[i] = cc_c['top_thickness']
     if(face_thickness[i]<2*cc_c['cross_cube_cnc_router_bit_radius']):
-      print("ERR244: Error, face_thickness[{:d}] {:0.3f} is too small compare to cross_cube_cnc_router_bit_radius {:0.3f}".format(i, face_thickness[i], cc_c['cross_cube_cnc_router_bit_radius']))
+      six.print_(("ERR244: Error, face_thickness[{:d}] {:0.3f} is too small compare to cross_cube_cnc_router_bit_radius {:0.3f}".format(i, face_thickness[i], cc_c['cross_cube_cnc_router_bit_radius'])))
       sys.exit(2)
     if(face_thickness[i]>cc_c['cube_width']/2.0-cc_c['axle_radius']):
-      print("ERR247: Error, face_thickness[{:d}] {:0.3f} is too big compare to cube_width {:0.3f} and axle_radius {:0.3f}".format(i, face_thickness[i], cc_c['cube_width'], cc_c['axle_radius']))
+      six.print_(("ERR247: Error, face_thickness[{:d}] {:0.3f} is too big compare to cube_width {:0.3f} and axle_radius {:0.3f}".format(i, face_thickness[i], cc_c['cube_width'], cc_c['axle_radius'])))
       sys.exit(2)
     if(face_thickness[i]>cc_c['cube_width']/6.0):
-      print("ERR250: Error, face_thickness[{:d}] {:0.3f} is too big compare to cube_width {:0.3f}".format(i, face_thickness[i], cc_c['cube_width']))
+      six.print_(("ERR250: Error, face_thickness[{:d}] {:0.3f} is too big compare to cube_width {:0.3f}".format(i, face_thickness[i], cc_c['cube_width'])))
       sys.exit(2)
   cc_c['face_A1_thickness'] = face_thickness[0]
   cc_c['face_A2_thickness'] = face_thickness[1]
@@ -219,61 +220,61 @@ def cross_cube_sub_face_check(ai_constraints):
   if(cc_c['face_rod_hole_radius']>0):
     # face_rod_hole_h_position
     if(cc_c['face_rod_hole_h_position']<cc_c['face_rod_hole_radius']):
-      print("ERR257: Error, face_rod_hole_h_position {:0.3f} must be biggert than face_rod_hole_radius {:0.3f}".format(cc_c['face_rod_hole_h_position'], cc_c['face_rod_hole_radius']))
+      six.print_(("ERR257: Error, face_rod_hole_h_position {:0.3f} must be biggert than face_rod_hole_radius {:0.3f}".format(cc_c['face_rod_hole_h_position'], cc_c['face_rod_hole_radius'])))
       sys.exit(2)
     if(cc_c['max_face_thickness']+cc_c['face_rod_hole_h_position']+cc_c['face_rod_hole_radius']>cc_c['cube_width']/2.0-cc_c['axle_radius']):
-      print("ERR261: Error, face_rod_hole_h_position {:0.3f} is too big compare to max_face_thickness {:0.3f}, face_rod_hole_radius {:0.3f}, cube_width {:0.3f}, axle_radius {:0.3f}".format(cc_c['face_rod_hole_h_position'], cc_c['max_face_thickness'], cc_c['face_rod_hole_radius'], cc_c['cube_width'], cc_c['axle_radius']))
+      six.print_(("ERR261: Error, face_rod_hole_h_position {:0.3f} is too big compare to max_face_thickness {:0.3f}, face_rod_hole_radius {:0.3f}, cube_width {:0.3f}, axle_radius {:0.3f}".format(cc_c['face_rod_hole_h_position'], cc_c['max_face_thickness'], cc_c['face_rod_hole_radius'], cc_c['cube_width'], cc_c['axle_radius'])))
       sys.exit(2)
     # face_rod_hole_v_distance
     if(cc_c['face_rod_hole_v_distance']<2*cc_c['face_rod_hole_radius']):
-      print("ERR264: Error, face_rod_hole_v_distance {:0.3f} must be bigger than face_rod_hole_diameter {:0.3f}".format(cc_c['face_rod_hole_v_distance'], 2*cc_c['face_rod_hole_radius']))
+      six.print_(("ERR264: Error, face_rod_hole_v_distance {:0.3f} must be bigger than face_rod_hole_diameter {:0.3f}".format(cc_c['face_rod_hole_v_distance'], 2*cc_c['face_rod_hole_radius'])))
       sys.exit(2)
     if(cc_c['face_rod_hole_v_position']+cc_c['face_rod_hole_v_distance']+cc_c['face_rod_hole_radius']>cc_c['height_margin']+cc_c['inter_axle_length']):
-      print("ERR296: Error, face_rod_hole_v_distance {:0.3f} is too big compare to face_rod_hole_v_position {:0.3f}, face_rod_hole_radius {:0.3f}, height_margin {:0.3f} and inter_axle_length {:0.3f}".format(cc_c['face_rod_hole_v_distance'], cc_c['face_rod_hole_v_position'], cc_c['face_rod_hole_radius'], cc_c['height_margin'], cc_c['inter_axle_length']))
+      six.print_(("ERR296: Error, face_rod_hole_v_distance {:0.3f} is too big compare to face_rod_hole_v_position {:0.3f}, face_rod_hole_radius {:0.3f}, height_margin {:0.3f} and inter_axle_length {:0.3f}".format(cc_c['face_rod_hole_v_distance'], cc_c['face_rod_hole_v_position'], cc_c['face_rod_hole_radius'], cc_c['height_margin'], cc_c['inter_axle_length'])))
       sys.exit(2)
     # face_rod_hole_v_position
     if(cc_c['face_rod_hole_v_position']<cc_c['face_rod_hole_radius']):
-      print("ERR297: Error, face_rod_hole_v_position {:0.3f} must be bigger than face_rod_hole_radius {:0.3f}".format(cc_c['face_rod_hole_v_position'], cc_c['face_rod_hole_radius']))
+      six.print_(("ERR297: Error, face_rod_hole_v_position {:0.3f} must be bigger than face_rod_hole_radius {:0.3f}".format(cc_c['face_rod_hole_v_position'], cc_c['face_rod_hole_radius'])))
       sys.exit(2)
     if(cc_c['face_rod_hole_v_position']+cc_c['face_rod_hole_radius']>cc_c['height_margin']):
-      print("ERR267: Error, face_rod_hole_v_position {:0.3f} is too big compare to face_rod_hole_radius {:0.3f} and height_margin {:0.3f}".format(cc_c['face_rod_hole_v_position'], cc_c['face_rod_hole_radius'], cc_c['height_margin']))
+      six.print_(("ERR267: Error, face_rod_hole_v_position {:0.3f} is too big compare to face_rod_hole_radius {:0.3f} and height_margin {:0.3f}".format(cc_c['face_rod_hole_v_position'], cc_c['face_rod_hole_radius'], cc_c['height_margin'])))
       sys.exit(2)
   ## top
   ### hollow
   ## face hollow
   # face_hollow_leg_nb
   if(not(cc_c['face_hollow_leg_nb'] in [1, 4, 8])):
-    print("ERR281: Error, face_hollow_leg_nb {:d} accepts only the values: 1, 4 or 8".format(cc_c['face_hollow_leg_nb']))
+    six.print_(("ERR281: Error, face_hollow_leg_nb {:d} accepts only the values: 1, 4 or 8".format(cc_c['face_hollow_leg_nb'])))
     sys.exit(2)
   # face_hollow_border_width
   if(cc_c['face_hollow_border_width']==0):
     cc_c['face_hollow_border_width'] = cc_c['max_face_top_thickness']
   if(cc_c['face_hollow_border_width']<radian_epsilon):
-    print("ERR283: Error, face_hollow_border_width {:0.3f} must be strictly positive".format(cc_c['face_hollow_border_width']))
+    six.print_(("ERR283: Error, face_hollow_border_width {:0.3f} must be strictly positive".format(cc_c['face_hollow_border_width'])))
     sys.exit(2)
   # face_hollow_axle_width
   if(cc_c['face_hollow_axle_width']==0):
     cc_c['face_hollow_axle_width'] = cc_c['axle_radius']
   if(cc_c['face_hollow_axle_width']<radian_epsilon):
-    print("ERR292: Error, face_hollow_axle_width {:0.3f} must be strictly positive".format(cc_c['face_hollow_axle_width']))
+    six.print_(("ERR292: Error, face_hollow_axle_width {:0.3f} must be strictly positive".format(cc_c['face_hollow_axle_width'])))
     sys.exit(2)
   # face_hollow_leg_width
   if(cc_c['face_hollow_leg_width']==0):
     cc_c['face_hollow_leg_width'] = max(cc_c['face_hollow_border_width'], cc_c['face_hollow_axle_width'])
   if(cc_c['face_hollow_leg_width']<radian_epsilon):
-    print("ERR297: Error, face_hollow_leg_width {:0.3f} must be strictly positive".format(cc_c['face_hollow_leg_width']))
+    six.print_(("ERR297: Error, face_hollow_leg_width {:0.3f} must be strictly positive".format(cc_c['face_hollow_leg_width'])))
     sys.exit(2)
   # face_hollow_smoothing_radius
   if(cc_c['face_hollow_smoothing_radius']==0):
     cc_c['face_hollow_smoothing_radius'] = cc_c['cube_width']/10.0
   if(cc_c['face_hollow_smoothing_radius']<cc_c['cross_cube_cnc_router_bit_radius']):
-    print("ERR302: Error, face_hollow_smoothing_radius {:0.3f} must be bigger than cross_cube_cnc_router_bit_radius {:0.3f}".format(cc_c['face_hollow_smoothing_radius'], cc_c['cross_cube_cnc_router_bit_radius']))
+    six.print_(("ERR302: Error, face_hollow_smoothing_radius {:0.3f} must be bigger than cross_cube_cnc_router_bit_radius {:0.3f}".format(cc_c['face_hollow_smoothing_radius'], cc_c['cross_cube_cnc_router_bit_radius'])))
     sys.exit(2)
   ### manufacturing
   # cross_cube_cnc_router_bit_radius
   # cross_cube_extra_cut_thickness
   if(abs(cc_c['cross_cube_extra_cut_thickness'])>min(cc_c['cube_width']/5.0, cc_c['cube_height']/4.0)/3.0):
-    print("ERR369: Error, cross_cube_extra_cut_thickness {:0.3} absolute value is too big compare to cube_width {:0.3f} and cube_height {:0.3f}".format(cc_c['cross_cube_extra_cut_thickness'], cc_c['cube_width'], cc_c['cube_height']))
+    six.print_(("ERR369: Error, cross_cube_extra_cut_thickness {:0.3} absolute value is too big compare to cube_width {:0.3f} and cube_height {:0.3f}".format(cc_c['cross_cube_extra_cut_thickness'], cc_c['cube_width'], cc_c['cube_height'])))
     sys.exit(2)
   # return
   return(cc_c)
@@ -294,55 +295,55 @@ def cross_cube_sub_top_check(ai_constraints):
   if(cc_c['top_rod_hole_radius']>0):
     # top_rod_hole_h_position
     if(cc_c['top_rod_hole_h_position']<cc_c['face_rod_hole_h_position']+cc_c['face_rod_hole_radius']+cc_c['top_rod_hole_radius']):
-      print("ERR273: Error, top_rod_hole_h_position {:0.3f} is too small compare to face_rod_hole_h_position {:0.3f}, face_rod_hole_radius {:0.3f} and top_rod_hole_radius {:0.3f}".format(cc_c['top_rod_hole_h_position'], cc_c['face_rod_hole_h_position'], cc_c['face_rod_hole_radius'], cc_c['top_rod_hole_radius']))
+      six.print_(("ERR273: Error, top_rod_hole_h_position {:0.3f} is too small compare to face_rod_hole_h_position {:0.3f}, face_rod_hole_radius {:0.3f} and top_rod_hole_radius {:0.3f}".format(cc_c['top_rod_hole_h_position'], cc_c['face_rod_hole_h_position'], cc_c['face_rod_hole_radius'], cc_c['top_rod_hole_radius'])))
       sys.exit(2)
     if(cc_c['max_face_thickness']+cc_c['top_rod_hole_h_position']+cc_c['top_rod_hole_radius']>cc_c['cube_width']/2.0-cc_c['axle_radius']):
-      print("ERR276: Error, top_rod_hole_h_position {:0.3f} is too big compare to max_face_thickness {:0.3f}, top_rod_hole_radius {:0.3f}, cube_width {:0.3f} and axle_radius {:0.3f}".format(cc_c['top_rod_hole_h_position'], cc_c['max_face_thickness'], cc_c['top_rod_hole_radius'], cc_c['cube_width'], cc_c['axle_radius']))
+      six.print_(("ERR276: Error, top_rod_hole_h_position {:0.3f} is too big compare to max_face_thickness {:0.3f}, top_rod_hole_radius {:0.3f}, cube_width {:0.3f} and axle_radius {:0.3f}".format(cc_c['top_rod_hole_h_position'], cc_c['max_face_thickness'], cc_c['top_rod_hole_radius'], cc_c['cube_width'], cc_c['axle_radius'])))
       sys.exit(2)
   ## top hollow
   # top_hollow_leg_nb
   if(not(cc_c['top_hollow_leg_nb'] in [0, 1, 4, 8])):
-    print("ERR325: Error, top_hollow_leg_nb {:d} accepts only the values: 0, 1, 4, 8".format(cc_c['top_hollow_leg_nb']))
+    six.print_(("ERR325: Error, top_hollow_leg_nb {:d} accepts only the values: 0, 1, 4, 8".format(cc_c['top_hollow_leg_nb'])))
     sys.exit(2)
   # top_hollow_border_width
   if(cc_c['top_hollow_border_width']==0):
     cc_c['top_hollow_border_width'] = cc_c['max_face_thickness']
   if(cc_c['top_hollow_border_width']<radian_epsilon):
-    print("ERR332: Error, top_hollow_border_width {:0.3f} must be strictly positive".format(cc_c['top_hollow_border_width']))
+    six.print_(("ERR332: Error, top_hollow_border_width {:0.3f} must be strictly positive".format(cc_c['top_hollow_border_width'])))
     sys.exit(2)
   # top_hollow_leg_width
   if(cc_c['top_hollow_leg_width']==0):
     cc_c['top_hollow_leg_width'] = cc_c['top_hollow_border_width']
   if(cc_c['top_hollow_leg_width']<radian_epsilon):
-    print("ERR338: Error, top_hollow_leg_width {:0.3f} must be strictly positive".format(cc_c['top_hollow_leg_width']))
+    six.print_(("ERR338: Error, top_hollow_leg_width {:0.3f} must be strictly positive".format(cc_c['top_hollow_leg_width'])))
     sys.exit(2)
   # top_hollow_smoothing_radius
   if(cc_c['top_hollow_smoothing_radius']==0):
     cc_c['top_hollow_smoothing_radius'] = cc_c['cube_width']/10.0
   if(cc_c['top_hollow_smoothing_radius']<cc_c['cross_cube_cnc_router_bit_radius']):
-    print("ERR344: Error, top_hollow_smoothing_radius {:0.3f} must be bigger than cross_cube_cnc_router_bit_radius {:0.3f}".format(cc_c['top_hollow_smoothing_radius'], cc_c['cross_cube_cnc_router_bit_radius']))
+    six.print_(("ERR344: Error, top_hollow_smoothing_radius {:0.3f} must be bigger than cross_cube_cnc_router_bit_radius {:0.3f}".format(cc_c['top_hollow_smoothing_radius'], cc_c['cross_cube_cnc_router_bit_radius'])))
     sys.exit(2)
   ### axle
   # axle_length
   if(cc_c['axle_length']==0):
     cc_c['axle_length'] = 2*cc_c['cube_width']
   if(cc_c['axle_length']<cc_c['cube_width']):
-    print("ERR349: Error, axle_length {:0.3f} must be bigger than cube_width {:0.3f}".format(cc_c['axle_length'], cc_c['cube_width']))
+    six.print_(("ERR349: Error, axle_length {:0.3f} must be bigger than cube_width {:0.3f}".format(cc_c['axle_length'], cc_c['cube_width'])))
     sys.exit(2)
   # spacer_diameter
   cc_c['spacer_radius'] = cc_c['spacer_diameter']/2.0
   if(cc_c['spacer_radius']==0):
     cc_c['spacer_radius'] = 1.2*cc_c['axle_radius']
   if(cc_c['spacer_radius']<cc_c['axle_radius']+radian_epsilon):
-    print("ERR357: Error, spacer_radius {:0.3f} must be bigger than axle_radius {:0.3f}".format(cc_c['spacer_radius'], cc_c['axle_radius']))
+    six.print_(("ERR357: Error, spacer_radius {:0.3f} must be bigger than axle_radius {:0.3f}".format(cc_c['spacer_radius'], cc_c['axle_radius'])))
     sys.exit(2)
   if(cc_c['spacer_length']==0):
     cc_c['spacer_length'] = (cc_c['axle_length']-cc_c['cube_width'])/4.0
   if(cc_c['spacer_length']<radian_epsilon):
-    print("ERR360: Error, spacer_length {:0.3f} must be strictly positive".format(cc_c['spacer_length']))
+    six.print_(("ERR360: Error, spacer_length {:0.3f} must be strictly positive".format(cc_c['spacer_length'])))
     sys.exit(2)
   if(cc_c['spacer_length']>(cc_c['axle_length']-cc_c['cube_width'])/2.0):
-    print("ERR363: Error, spacer_length {:0.3f} is too big compare to axle_length {:0.3f} and cube_width {:0.3f}".format(cc_c['spacer_length'], cc_c['axle_length'], cc_c['cube_width']))
+    six.print_(("ERR363: Error, spacer_length {:0.3f} is too big compare to axle_length {:0.3f} and cube_width {:0.3f}".format(cc_c['spacer_length'], cc_c['axle_length'], cc_c['cube_width'])))
     sys.exit(2)
   # return
   return(cc_c)
diff --git a/cnc25d/design_frontend.py b/cnc25d/design_frontend.py
index badae5f..0ab5636 100644
--- a/cnc25d/design_frontend.py
+++ b/cnc25d/design_frontend.py
@@ -43,6 +43,7 @@
 import math
 import sys
 import re
+import six
 #import argparse
 #from datetime import datetime
 #import os, errno
@@ -75,7 +76,7 @@ def check(constraint_dict={}, error_id="ERR000", error_msg='', constraint_dict_n
       msg_introduction = "Warning on"
     #print("ERR073: Error on {:s} with:".format(error_msg))
     error_msg_without_dict = re.sub("'\]", " ", re.sub("{:s}\['".format(constraint_dict_name), " ", error_msg))
-    print("{:s}: {:s} {:s} with:".format(error_id, msg_introduction, error_msg_without_dict))
+    six.print_(("{:s}: {:s} {:s} with:".format(error_id, msg_introduction, error_msg_without_dict)))
     var_list = re.findall("{:s}\['\w+'\]".format(constraint_dict_name), error_msg)
     #print("dbg075: var_list:", var_list)
     for d_item in var_list:
@@ -83,7 +84,7 @@ def check(constraint_dict={}, error_id="ERR000", error_msg='', constraint_dict_n
       #d_key = re.sub("^.*\['(\w+)'\]$", "\1", d_item)
       #print("dbg078: d_key:", d_key)
       d_val = constraint_dict[d_key]
-      print("{:s}['{:s}'] = {:s}".format(constraint_dict_name, d_key, str(d_val)))
+      six.print_(("{:s}['{:s}'] = {:s}".format(constraint_dict_name, d_key, str(d_val))))
     if(not warning_nerror):
       sys.exit(2)
     else:
diff --git a/cnc25d/design_help.py b/cnc25d/design_help.py
index 5fea92a..223a51c 100644
--- a/cnc25d/design_help.py
+++ b/cnc25d/design_help.py
@@ -44,6 +44,7 @@
 import sys, argparse
 #from datetime import datetime
 import os, errno
+import six
 #import re
 #import Tkinter # to display the outline in a small GUI
 # FreeCAD
@@ -72,7 +73,7 @@ def mkdir_p(ai_directory):
   if(ai_directory!=''):
     #print("dbg448: ai_directory:", ai_directory)
     # mkdir -p directory if needed
-    print("dbg450: try to create the output directory: {:s}".format(ai_directory))
+    six.print_(("dbg450: try to create the output directory: {:s}".format(ai_directory)))
     try:
       os.makedirs(ai_directory)
       r_status = 0
diff --git a/cnc25d/design_output.py b/cnc25d/design_output.py
index 024617f..7a82309 100644
--- a/cnc25d/design_output.py
+++ b/cnc25d/design_output.py
@@ -29,7 +29,8 @@
 # header for Python / FreeCAD compatibility
 ################################################################
 
-import importing_freecad
+from . import importing_freecad
+import six
 importing_freecad.importing_freecad()
 
 #print("FreeCAD.Version:", FreeCAD.Version())
@@ -54,11 +55,11 @@
 #import svgwrite
 #from dxfwrite import DXFEngine
 # cnc25d
-import outline_backends
-import export_2d
-import design_help
-import cnc_outline
-import positioning
+from . import outline_backends
+from . import export_2d
+from . import design_help
+from . import cnc_outline
+from . import positioning
 
 
 ################################################################
@@ -114,21 +115,21 @@ def generate_output_file(ai_figure, ai_output_filename, ai_height, ai_info_txt='
       outline_backends.write_figure_in_svg(ai_figure, ai_output_filename)
     # FreeCAD
     elif(re.search('\.brep$', ai_output_filename)):
-      print("Generate with FreeCAD the BRep file {:s}".format(ai_output_filename))
+      six.print_(("Generate with FreeCAD the BRep file {:s}".format(ai_output_filename)))
       freecad_part = outline_backends.figure_to_freecad_25d_part(ai_figure, ai_height)
       freecad_part.exportBrep("{:s}".format(ai_output_filename))
-      print("Generate with FreeCAD the DXF file {:s}.dxf".format(ai_output_filename))
+      six.print_(("Generate with FreeCAD the DXF file {:s}.dxf".format(ai_output_filename)))
       # slice freecad_part  in the XY plan at a height of ai_height/2
       export_2d.export_to_dxf(freecad_part, Base.Vector(0,0,1), ai_height/2, "{:s}.dxf".format(ai_output_filename))
     elif(re.search('\.stl$', ai_output_filename)):
-      print("Generate with FreeCAD the STL file {:s}".format(ai_output_filename))
+      six.print_(("Generate with FreeCAD the STL file {:s}".format(ai_output_filename)))
       freecad_part = outline_backends.figure_to_freecad_25d_part(ai_figure, ai_height)
       freecad_part.exportStl("{:s}".format(ai_output_filename))
-      print("Generate with FreeCAD the DXF file {:s}.dxf".format(ai_output_filename))
+      six.print_(("Generate with FreeCAD the DXF file {:s}.dxf".format(ai_output_filename)))
       # slice freecad_part  in the XY plan at a height of ai_height/2
       export_2d.export_to_dxf(freecad_part, Base.Vector(0,0,1), ai_height/2, "{:s}.dxf".format(ai_output_filename))
     else:
-      print("ERR124: Error: the suffix of the filename {:s} is unknown. Try with suffix: .dxf, .svg, .brep or .stl".format(ai_output_filename))
+      six.print_(("ERR124: Error: the suffix of the filename {:s} is unknown. Try with suffix: .dxf, .svg, .brep or .stl".format(ai_output_filename)))
       sys.exit(2)
     # info_txt
     #if(ai_info_txt!=''):
@@ -147,15 +148,15 @@ def freecad_object_output_file(ai_freecad_object, ai_output_filename, ai_brep=Tr
   """
   if(ai_brep):
     brep_output_filename = "{:s}.brep".format(ai_output_filename)
-    print("Generate with FreeCAD the BRep file {:s}".format(brep_output_filename))
+    six.print_(("Generate with FreeCAD the BRep file {:s}".format(brep_output_filename)))
     ai_freecad_object.exportBrep(brep_output_filename)
   if(ai_stl):
     stl_output_filename = "{:s}.stl".format(ai_output_filename)
-    print("Generate with FreeCAD the STL file {:s}".format(stl_output_filename))
+    six.print_(("Generate with FreeCAD the STL file {:s}".format(stl_output_filename)))
     ai_freecad_object.exportStl(stl_output_filename)
   if(len(ai_slice_xyz)>0):
     if(len(ai_slice_xyz)!=9):
-      print("ERR150: Error, len(ai_slice_xyz) {:d} must be 9".format(len(ai_slice_xyz)))
+      six.print_(("ERR150: Error, len(ai_slice_xyz) {:d} must be 9".format(len(ai_slice_xyz))))
       sys.exit(2)
     size_x = ai_slice_xyz[0]
     size_y = ai_slice_xyz[1]
@@ -167,7 +168,7 @@ def freecad_object_output_file(ai_freecad_object, ai_output_filename, ai_brep=Tr
     slice_y = ai_slice_xyz[7]
     slice_z = ai_slice_xyz[8]
     dxf_output_filename = "{:s}_xyz_slices.dxf".format(ai_output_filename)
-    print("Slice with FreeCAD the 3D into the DXF file {:s}".format(dxf_output_filename))
+    six.print_(("Slice with FreeCAD the 3D into the DXF file {:s}".format(dxf_output_filename)))
     #print("dbg161: zero_x {:0.3f}  zero_y {:0.3f}  zero_z {:0.3f}".format(zero_x, zero_y, zero_z))
     ai_freecad_object.translate(Base.Vector(-1*zero_x, -1*zero_y, -1*zero_z))
     export_2d.export_xyz_to_dxf(ai_freecad_object, size_x, size_y, size_z, slice_x, slice_y, slice_z, dxf_output_filename)
@@ -176,7 +177,7 @@ def freecad_object_output_file(ai_freecad_object, ai_output_filename, ai_brep=Tr
 def generate_3d_assembly_output_file(ai_3d_conf, ai_output_filename, ai_brep=True, ai_stl=False, ai_slice_xyz=[]):
   """ implement the swith --output_file_basename for 3D assembly
   """
-  print("Compute with FreeCAD the 3D assembly {:s}".format(ai_output_filename))
+  six.print_(("Compute with FreeCAD the 3D assembly {:s}".format(ai_output_filename)))
   fc_assembly = figures_to_freecad_assembly(ai_3d_conf)
   freecad_object_output_file(fc_assembly, ai_output_filename, ai_brep, ai_stl, ai_slice_xyz)
   return(0)
@@ -237,7 +238,7 @@ def figures_to_freecad_assembly(ai_figure_assembly):
   fc_obj = []
   for i in range(obj_nb):
     if(len(ai_figure_assembly[i])!=11):
-      print("ERR219: Error len of ai_figure_assembly {:d} must be 11".format(len(ai_figure_assembly[i])))
+      six.print_(("ERR219: Error len of ai_figure_assembly {:d} must be 11".format(len(ai_figure_assembly[i]))))
       sys.exit(2)
     (part_figure, zero_x, zero_y, size_x, size_y, size_z, flip, orientation, translate_x, translate_y, translate_z) = ai_figure_assembly[i]
     part_figure_zero = rotate_and_translate_figure(part_figure, 0, 0, 0, -1*zero_x, -1*zero_y)
diff --git a/cnc25d/display_backend.py b/cnc25d/display_backend.py
index 0932d52..c6b0d80 100644
--- a/cnc25d/display_backend.py
+++ b/cnc25d/display_backend.py
@@ -33,10 +33,11 @@
 
 import math
 import sys, argparse
-import Tkinter
+from six.moves import tkinter
 #import tkMessageBox
 import matplotlib.pyplot
-import design_help # just for get_effective_args()
+from . import design_help # just for get_effective_args()
+import six
 
 ################################################################
 # global variable
@@ -69,7 +70,7 @@ def find_outline_extremum(ai_outline_list):
     first_point_x=first_outline[0]
     first_point_y=first_outline[1]
   else:
-    print("ERR305: Error, the outline type is unknow: {:s}".format(first_outline_type))
+    six.print_(("ERR305: Error, the outline type is unknow: {:s}".format(first_outline_type)))
     sys.exit(2)
   min_x=first_point_x
   max_x=first_point_x
@@ -291,11 +292,11 @@ def action_button_check_event(self, event):
     """ This function is only used for debug
         It has no utility for the Two_Canvas class
     """
-    print("dbg141: event:", event)
-    print("dbg142: event.time:", event.time)
-    print("dbg143: event.type:", event.type)
-    print("dbg144: event.widget:", event.widget)
-    print("dbg145: event.keysym:", event.keysym)
+    six.print_(("dbg141: event:", event))
+    six.print_(("dbg142: event.time:", event.time))
+    six.print_(("dbg143: event.type:", event.type))
+    six.print_(("dbg144: event.widget:", event.widget))
+    six.print_(("dbg145: event.keysym:", event.keysym))
 
   def action_canvas_a_mouse_button_press(self, event):
     """ Start of the zoom area selection
@@ -366,8 +367,8 @@ def action_canvas_b_mouse_button_release(self, event):
       if(length_measurement>0):
         angle_measurement = math.atan2((p2y-p1y)/length_measurement, (p2x-p1x)/length_measurement)
         self.measurement_id += 1
-        print("Info: measurement {:2d} : lenght: {:0.3f}  angle: {:0.3f}".format(self.measurement_id, length_measurement, angle_measurement))
-        print("P1x: {:0.2f}  P1y: {:0.2f}  P2x: {:0.2f}  P2y: {:0.2f}".format(p1x, p1y, p2x, p2y))
+        six.print_(("Info: measurement {:2d} : lenght: {:0.3f}  angle: {:0.3f}".format(self.measurement_id, length_measurement, angle_measurement)))
+        six.print_(("P1x: {:0.2f}  P1y: {:0.2f}  P2x: {:0.2f}  P2y: {:0.2f}".format(p1x, p1y, p2x, p2y)))
 
   def simulation_step(self):
     """ Time simulation main function
@@ -414,7 +415,7 @@ def apply_canvas_graphic_function(self):
     if(self.canvas_graphic_function!=None):
       all_graphics = self.canvas_graphic_function(self.rotation_direction, self.angle_position)
       #
-      self.canvas_a.delete(Tkinter.ALL)
+      self.canvas_a.delete(tkinter.ALL)
       # uncomment if you want to scale outline depending on the angle_position
       #self.outline_extremum = find_outline_extremum(all_graphics)
       canvas_a_size = (canvas_a_width, canvas_a_height, tkinter_canvas_margin_x, tkinter_canvas_margin_y)
@@ -427,7 +428,7 @@ def apply_canvas_graphic_function(self):
       if(self.crop_limit==(0,0,0,0)):
         self.crop_limit=(self.outline_extremum[0], self.outline_extremum[1], (self.outline_extremum[0]+self.outline_extremum[2])/2, (self.outline_extremum[1]+self.outline_extremum[3])/2)
       #
-      self.canvas_b.delete(Tkinter.ALL)
+      self.canvas_b.delete(tkinter.ALL)
       crop_graphics = crop_outline(all_graphics, self.crop_limit)
       #print("dbg857: len(crop_graphics):", len(crop_graphics))
       canvas_b_size = (canvas_b_width, canvas_b_height, 2, 2)
@@ -465,15 +466,15 @@ def create_zoom_frame(self):
     """ Define the zoom window
     """
     #print("dbg554: create_zoom_frame")
-    self.frame_b = Tkinter.Toplevel(self.frame_a)
+    self.frame_b = tkinter.Toplevel(self.frame_a)
     #self.frame_b.grid(column=0, row=0, sticky=Tkinter.N+Tkinter.E+Tkinter.S+Tkinter.W) # Toplevel doesn't have grid method !
     #self.frame_b.pack(fill=Tkinter.BOTH, expand=1)
     self.frame_b.title("cnc25d display backend details")
-    self.canvas_b = Tkinter.Canvas(self.frame_b, width=initial_tkinter_canvas_width, height=initial_tkinter_canvas_height)
+    self.canvas_b = tkinter.Canvas(self.frame_b, width=initial_tkinter_canvas_width, height=initial_tkinter_canvas_height)
     #self.canvas_b.grid(column=0, row=0, sticky=Tkinter.N+Tkinter.E+Tkinter.S+Tkinter.W)
     #self.canvas_b.columnconfigure(0, weight=1)
     #self.canvas_b.rowconfigure(0, weight=1)
-    self.canvas_b.pack(fill=Tkinter.BOTH, expand=1) # with Toplevel parent, it seems you need to use pack to resisze the canvas !
+    self.canvas_b.pack(fill=tkinter.BOTH, expand=1) # with Toplevel parent, it seems you need to use pack to resisze the canvas !
     self.canvas_b.bind("<ButtonPress-1>", self.action_canvas_b_mouse_button_press)
     self.canvas_b.bind("<B1-Motion>", self.action_canvas_b_mouse_button_motion)
     self.canvas_b.bind("<ButtonRelease-1>", self.action_canvas_b_mouse_button_release)
@@ -510,10 +511,10 @@ def create_parameter_frame(self):
     """ Define the parameter window
     """
     #print("dbg414: create_parameter_frame")
-    self.frame_c = Tkinter.Toplevel(self.frame_a)
+    self.frame_c = tkinter.Toplevel(self.frame_a)
     self.frame_c.title("parameter info")
-    self.parameter_message = Tkinter.Message(self.frame_c, textvariable=self.parameter_content)
-    self.parameter_message.grid(sticky=Tkinter.N+Tkinter.E+Tkinter.S+Tkinter.W)
+    self.parameter_message = tkinter.Message(self.frame_c, textvariable=self.parameter_content)
+    self.parameter_message.grid(sticky=tkinter.N+tkinter.E+tkinter.S+tkinter.W)
     self.frame_c.protocol("WM_DELETE_WINDOW", self.hide_parameter_frame) # change the behaviour of the window X button
 
   def hide_parameter_frame(self):
@@ -561,7 +562,7 @@ def action_button_curve_graph(self, event):
       for j in range(curve_nb-1):
         check_len = len( self.curve_graphic_table[j+2][1])
         if(check_len!=curve_table_len):
-          print("ERR586: Error in the curve_table {:d}! Its lenght {:d} does not match the reference length {:d}".format(j, check_len, curve_table_len))
+          six.print_(("ERR586: Error in the curve_table {:d}! Its lenght {:d} does not match the reference length {:d}".format(j, check_len, curve_table_len)))
       x_increment = self.curve_graphic_table[0][2]
       abscissa_x = 0
       for i in range(curve_table_len):
@@ -604,105 +605,105 @@ def createWidgets(self):
     #self.frame_canvas.rowconfigure(0, weight=1)
     #
     #self.canvas_a =  Tkinter.Canvas(self.frame_a, width=self.canvas_a_width, height=self.canvas_a_height)
-    self.canvas_a =  Tkinter.Canvas(self.frame_a, width=initial_tkinter_canvas_width, height=initial_tkinter_canvas_height)
+    self.canvas_a =  tkinter.Canvas(self.frame_a, width=initial_tkinter_canvas_width, height=initial_tkinter_canvas_height)
     #self.canvas_a.pack(fill=Tkinter.BOTH, expand=1)
-    self.canvas_a.grid(column=0, row=0, sticky=Tkinter.N+Tkinter.E+Tkinter.S+Tkinter.W)
+    self.canvas_a.grid(column=0, row=0, sticky=tkinter.N+tkinter.E+tkinter.S+tkinter.W)
     self.canvas_a.columnconfigure(0, weight=1)
     self.canvas_a.rowconfigure(0, weight=1)
     self.canvas_a.bind("<ButtonPress-1>", self.action_canvas_a_mouse_button_press)
     self.canvas_a.bind("<B1-Motion>", self.action_canvas_a_mouse_button_motion)
     self.canvas_a.bind("<ButtonRelease-1>", self.action_canvas_a_mouse_button_release)
     #
-    self.frame_control = Tkinter.Frame(self.frame_a)
+    self.frame_control = tkinter.Frame(self.frame_a)
     #self.frame_control.pack(side=Tkinter.BOTTOM)
-    self.frame_control.grid(column=0, row=1, sticky=Tkinter.W + Tkinter.S)
+    self.frame_control.grid(column=0, row=1, sticky=tkinter.W + tkinter.S)
     #
-    self.frame_button_speed = Tkinter.Frame(self.frame_control)
+    self.frame_button_speed = tkinter.Frame(self.frame_control)
     self.frame_button_speed.grid(column=0, row=0)
     #
-    self.button_fast_backward = Tkinter.Button(self.frame_button_speed)
+    self.button_fast_backward = tkinter.Button(self.frame_button_speed)
     self.button_fast_backward["text"] = "<<-",
     self.button_fast_backward["command"] = self.action_button_fast_backward
-    self.button_fast_backward.pack(side=Tkinter.LEFT)
+    self.button_fast_backward.pack(side=tkinter.LEFT)
     #
-    self.button_slow_backward = Tkinter.Button(self.frame_button_speed)
+    self.button_slow_backward = tkinter.Button(self.frame_button_speed)
     self.button_slow_backward["text"] = "<-",
     self.button_slow_backward["command"] = self.action_button_slow_backward
-    self.button_slow_backward.pack(side=Tkinter.LEFT)
+    self.button_slow_backward.pack(side=tkinter.LEFT)
     #
-    self.button_step_backward = Tkinter.Button(self.frame_button_speed)
+    self.button_step_backward = tkinter.Button(self.frame_button_speed)
     self.button_step_backward["text"] = "<|",
     self.button_step_backward["command"] = self.action_button_step_backward
-    self.button_step_backward.pack(side=Tkinter.LEFT)
+    self.button_step_backward.pack(side=tkinter.LEFT)
     #
-    self.button_stop = Tkinter.Button(self.frame_button_speed)
+    self.button_stop = tkinter.Button(self.frame_button_speed)
     self.button_stop["foreground"]   = "red"
     self.button_stop["text"] = "||",
     self.button_stop["command"] = self.action_button_stop
-    self.button_stop.pack(side=Tkinter.LEFT)
+    self.button_stop.pack(side=tkinter.LEFT)
     #
-    self.button_step_forward = Tkinter.Button(self.frame_button_speed)
+    self.button_step_forward = tkinter.Button(self.frame_button_speed)
     self.button_step_forward["text"] = "|>",
     self.button_step_forward["command"] = self.action_button_step_forward
-    self.button_step_forward.pack(side=Tkinter.LEFT)
+    self.button_step_forward.pack(side=tkinter.LEFT)
     #
-    self.button_slow_forward = Tkinter.Button(self.frame_button_speed)
+    self.button_slow_forward = tkinter.Button(self.frame_button_speed)
     self.button_slow_forward["text"] = "->",
     self.button_slow_forward["command"] = self.action_button_slow_forward
-    self.button_slow_forward.pack(side=Tkinter.LEFT)
+    self.button_slow_forward.pack(side=tkinter.LEFT)
     #
-    self.button_fast_forward = Tkinter.Button(self.frame_button_speed)
+    self.button_fast_forward = tkinter.Button(self.frame_button_speed)
     self.button_fast_forward["text"] = "->>",
     self.button_fast_forward["command"] = self.action_button_fast_forward
-    self.button_fast_forward.pack(side=Tkinter.LEFT)
+    self.button_fast_forward.pack(side=tkinter.LEFT)
     #
-    self.frame_label = Tkinter.Frame(self.frame_control)
-    self.frame_label.grid(column=0, row=1, sticky=Tkinter.W)
+    self.frame_label = tkinter.Frame(self.frame_control)
+    self.frame_label.grid(column=0, row=1, sticky=tkinter.W)
     #
-    self.label_angle = Tkinter.Label(self.frame_label)
+    self.label_angle = tkinter.Label(self.frame_label)
     self.label_angle["textvariable"] = self.label1_content
     #self.label_angle.pack(side=Tkinter.TOP)
     #self.label_angle.pack(anchor=Tkinter.NW)
-    self.label_angle.grid(column=0, row=0, sticky=Tkinter.W)
+    self.label_angle.grid(column=0, row=0, sticky=tkinter.W)
     #
-    self.label_angle_speed = Tkinter.Label(self.frame_label)
+    self.label_angle_speed = tkinter.Label(self.frame_label)
     self.label_angle_speed["textvariable"] = self.label2_content
     #self.label_angle_speed.pack(side=Tkinter.BOTTOM)
     #self.label_angle_speed.pack(anchor=Tkinter.SW)
-    self.label_angle_speed.grid(column=0, row=1, sticky=Tkinter.W)
+    self.label_angle_speed.grid(column=0, row=1, sticky=tkinter.W)
     #
-    self.frame_button_options = Tkinter.Frame(self.frame_control)
-    self.frame_button_options.grid(column=0, row=2, sticky=Tkinter.W+Tkinter.S)
+    self.frame_button_options = tkinter.Frame(self.frame_control)
+    self.frame_button_options.grid(column=0, row=2, sticky=tkinter.W+tkinter.S)
     #
-    self.button_zoom = Tkinter.Button(self.frame_button_options)
+    self.button_zoom = tkinter.Button(self.frame_button_options)
     self.button_zoom["text"] = "Zoom",
     self.button_zoom["command"] = self.action_button_zoom
-    self.button_zoom.pack(side=Tkinter.LEFT)
+    self.button_zoom.pack(side=tkinter.LEFT)
     #
-    self.button_overlay = Tkinter.Button(self.frame_button_options)
+    self.button_overlay = tkinter.Button(self.frame_button_options)
     self.button_overlay["text"] = "Overlay",
     self.button_overlay["command"] = self.action_button_overlay
-    self.button_overlay.pack(side=Tkinter.LEFT)
+    self.button_overlay.pack(side=tkinter.LEFT)
     #
-    self.button_parameter = Tkinter.Button(self.frame_button_options)
+    self.button_parameter = tkinter.Button(self.frame_button_options)
     self.button_parameter["text"] = "Parameters",
     self.button_parameter["command"] = self.action_button_parameters
-    self.button_parameter.pack(side=Tkinter.LEFT)
+    self.button_parameter.pack(side=tkinter.LEFT)
     #
-    self.button_graph = Tkinter.Button(self.frame_button_options)
+    self.button_graph = tkinter.Button(self.frame_button_options)
     self.button_graph["text"] = "Graph",
     #self.button_graph["command"] = lambda a=1: self.action_button_check_event(a)
     #self.button_graph.bind("<Button-1>", lambda event: self.action_button_check_event(event))
     #self.button_graph.bind("<Button-1>", self.action_button_check_event) # the event argument is added by default
     self.button_graph.bind("<Button-1>", self.action_button_curve_graph) # the event argument is added by default
-    self.button_graph.pack(side=Tkinter.LEFT)
+    self.button_graph.pack(side=tkinter.LEFT)
     #
-    self.button_quit = Tkinter.Button(self.frame_button_options)
+    self.button_quit = tkinter.Button(self.frame_button_options)
     self.button_quit["text"] = "Quit",
     #self.button_quit["command"] = self.frame_a.quit
     #self.button_quit["command"] = self.tktop.destroy
     self.button_quit["command"] = self.quit_Two_Canvas
-    self.button_quit.pack(side=Tkinter.LEFT)
+    self.button_quit.pack(side=tkinter.LEFT)
     #
     ## second window with canvas_b
     self.create_zoom_frame()
@@ -716,13 +717,13 @@ def __init__(self, winParent):
     """ Initiate the class Two_Canvas by creating the windows, initializing the class variables and starting the time simulation
     """
     self.tktop = winParent # used to destroy the app when quit
-    self.frame_a = Tkinter.Frame(winParent)
+    self.frame_a = tkinter.Frame(winParent)
     winParent.title("cnc25d display backend main")
     #self.frame_a.pack()
     #winParent.grid(sticky=Tkinter.N+Tkinter.E+Tkinter.S+Tkinter.W)
     winParent.columnconfigure(0, weight=1)
     winParent.rowconfigure(0, weight=1)
-    self.frame_a.grid(sticky=Tkinter.N+Tkinter.E+Tkinter.S+Tkinter.W)
+    self.frame_a.grid(sticky=tkinter.N+tkinter.E+tkinter.S+tkinter.W)
     self.frame_a.columnconfigure(0, weight=1)
     self.frame_a.rowconfigure(0, weight=1)
     #
@@ -731,11 +732,11 @@ def __init__(self, winParent):
     self.angle_position = 0
     self.outline_extremum = (0,0,0,0)
     #
-    self.label1_content = Tkinter.StringVar()
-    self.label2_content = Tkinter.StringVar()
+    self.label1_content = tkinter.StringVar()
+    self.label2_content = tkinter.StringVar()
     self.set_label_content()
     #
-    self.parameter_content = Tkinter.StringVar()
+    self.parameter_content = tkinter.StringVar()
     #
     self.overlay = 0
     self.canvas_graphic_function = None
@@ -865,7 +866,7 @@ def two_canvas_class_test1():
     ('plot1_title', test1_curve1, 'bo'),
     ('plot2_title', test1_curve2, 'r'))
   #
-  tk_root = Tkinter.Tk()
+  tk_root = tkinter.Tk()
   dut = Two_Canvas(tk_root)
   #dut = Two_Canvas()
   dut.add_canvas_graphic_function(test_canvas_graphic_1)
@@ -909,7 +910,7 @@ def two_canvas_class_test2():
     ('plot1_title', test1_curve1, 'bo'),
     ('plot2_title', test1_curve2, 'r'))
   #
-  tk_root = Tkinter.Tk()
+  tk_root = tkinter.Tk()
   dut = Two_Canvas(tk_root)
   dut.add_canvas_graphic_function(test_canvas_graphic_2)
   dut.add_parameter_info(test_parameter_info)
diff --git a/cnc25d/draw_2d_frontend.py b/cnc25d/draw_2d_frontend.py
index 31cda1d..cca64a9 100644
--- a/cnc25d/draw_2d_frontend.py
+++ b/cnc25d/draw_2d_frontend.py
@@ -49,8 +49,9 @@
 #from datetime import datetime
 #import os, errno
 ## cnc25d
-import small_geometry
-import cnc_outline
+from . import small_geometry
+from . import cnc_outline
+import six
 #import outline_backends
 #import export_2d
 
@@ -65,10 +66,10 @@ def get_center(CX=0.0, CY=0.0, C=()):
   OCY = CY
   if(len(C)>0):
     if((CX!=0)or(CY!=0)):
-      print("ERR112: Error, CX {:0.3f} or CY {:0.3d} have been set simultaneously with C".format(CX, CY))
+      six.print_(("ERR112: Error, CX {:0.3f} or CY {:0.3d} have been set simultaneously with C".format(CX, CY)))
       sys.exit(2)
     if(len(C)!=2):
-      print("ERR115: len(C) {:d} is not 2".format(len(C)))
+      six.print_(("ERR115: len(C) {:d} is not 2".format(len(C))))
       sys.exit(2)
     OCX = C[0]
     OCY = C[1]
@@ -82,7 +83,7 @@ def c_xy(d, a, CX=0.0, CY=0.0, C=()):
   """
   # check distance
   if(d<0):
-    print("ERR104: Error, the polar coordinate distance d {:0.3f} is negative".format(d))
+    six.print_(("ERR104: Error, the polar coordinate distance d {:0.3f} is negative".format(d)))
     sys.exit(2)
   # origin-center
   (OCX, OCY) = get_center(CX, CY, C)
@@ -133,7 +134,7 @@ def add_StartPoint(self, CX=0.0, CY=0.0, C=(), rbr=0.0):
     rbr is the router_bit_radius to be applied on the StartPoint corner.
     """
     if(self.ol!=[]):
-      print("ERR152: Error, StartPoint can not be added to the outline {:s} with {:d} segments".format(self.outline_id, len(self.ol)))
+      six.print_(("ERR152: Error, StartPoint can not be added to the outline {:s} with {:d} segments".format(self.outline_id, len(self.ol))))
       sys.exit(2)
     (X, Y) = get_center(CX, CY, C)
     self.ol.append((X, Y, rbr))
@@ -144,7 +145,7 @@ def add_LineTo(self, CX=0.0, CY=0.0, C=(), rbr=0.0):
     rbr is the router_bit_radius to be applied on the new point corner.
     """
     if(len(self.ol)<1):
-      print("ERR163: Error, line-segment can not be added to the outline {:s} with {:d} segments. Add StartPoint first.".format(self.outline_id, len(self.ol)))
+      six.print_(("ERR163: Error, line-segment can not be added to the outline {:s} with {:d} segments. Add StartPoint first.".format(self.outline_id, len(self.ol))))
       sys.exit(2)
     (X, Y) = get_center(CX, CY, C)
     self.ol.append((X, Y, rbr))
@@ -155,7 +156,7 @@ def add_ArcThrTo(self, CX1=0.0, CY1=0.0, CX2=0.0, CY2=0.0, C1=(), C2=(), rbr=0.0
     rbr is the router_bit_radius to be applied on the CX2,CY2 corner.
     """
     if(len(self.ol)<1):
-      print("ERR174: Error, arc-segment can not be added to the outline {:s} with {:d} segments. Add StartPoint first.".format(self.outline_id, len(self.ol)))
+      six.print_(("ERR174: Error, arc-segment can not be added to the outline {:s} with {:d} segments. Add StartPoint first.".format(self.outline_id, len(self.ol))))
       sys.exit(2)
     (X1, Y1) = get_center(CX1, CY1, C1)
     (X2, Y2) = get_center(CX2, CY2, C2)
@@ -167,7 +168,7 @@ def add_ArcThrTo_radius_angles(self, R, a1, a2, CX=0.0, CY=0.0, C=(), rbr=0.0):
     rbr is the router_bit_radius to be applied on the CX2,CY2 corner.
     """
     if(len(self.ol)<1):
-      print("ERR174: Error, arc-segment can not be added to the outline {:s} with {:d} segments. Add StartPoint first.".format(self.outline_id, len(self.ol)))
+      six.print_(("ERR174: Error, arc-segment can not be added to the outline {:s} with {:d} segments. Add StartPoint first.".format(self.outline_id, len(self.ol))))
       sys.exit(2)
     (X1, Y1) = c_xy(R, a1, CX, CY, C)
     (X2, Y2) = c_xy(R, a2, CX, CY, C)
@@ -184,7 +185,7 @@ def close_with_Line(self):
     Close the outline with a line-segment
     """
     if(len(self.ol)<2):
-      print("ERR157: Error, line-segment can not be added to close the outline {:s} with {:d} segments. Add at least one segment first.".format(self.outline_id, len(self.ol)))
+      six.print_(("ERR157: Error, line-segment can not be added to close the outline {:s} with {:d} segments. Add at least one segment first.".format(self.outline_id, len(self.ol))))
       sys.exit(2)
     X = self.ol[0][0]
     Y = self.ol[0][1]
@@ -195,7 +196,7 @@ def close_with_ArcThr(self, CX1=0.0, CY1=0.0, C1=()):
     Close the outline with an arc-segment
     """
     if(len(self.ol)<2):
-      print("ERR168: Error, arc-segment can not be added to close the outline {:s} with {:d} segments. Add at least one segment first.".format(self.outline_id, len(self.ol)))
+      six.print_(("ERR168: Error, arc-segment can not be added to close the outline {:s} with {:d} segments. Add at least one segment first.".format(self.outline_id, len(self.ol))))
       sys.exit(2)
     (X1, Y1) = get_center(CX1, CY1, C1)
     X2 = self.ol[0][0]
@@ -210,7 +211,7 @@ def close_insurance(self):
     radian_epsilon = math.pi/1000
     #
     if((abs(self.ol[0][0]-self.ol[-1][-3])>radian_epsilon)or(abs(self.ol[0][1]-self.ol[-1][-2])>radian_epsilon)):
-      print("ERR206: Error, outline {:s} should be closed but is not closed: first point ({:0.3f}, {:0.3f}), last point ({:0.3f}, {:0.3f})".format(self.outline_id, self.ol[0][0], self.ol[0][1], self.ol[-1][-3], self.ol[-1][-2]))
+      six.print_(("ERR206: Error, outline {:s} should be closed but is not closed: first point ({:0.3f}, {:0.3f}), last point ({:0.3f}, {:0.3f})".format(self.outline_id, self.ol[0][0], self.ol[0][1], self.ol[-1][-3], self.ol[-1][-2])))
       sys.exit(2)
     last_segment = list(self.ol[-1])
     last_segment[-3] = self.ol[0][0]
@@ -250,20 +251,20 @@ def check(self, figure_id=""):
     radian_epsilon = math.pi/1000
     #
     if(len(self.ol)<2):
-      print("ERR179: Error, figure {:s}, outline {:s} with {:d} segments is too short. Add at leat two segment first.".format(figure_id, self.outline_id, len(self.ol)))
+      six.print_(("ERR179: Error, figure {:s}, outline {:s} with {:d} segments is too short. Add at leat two segment first.".format(figure_id, self.outline_id, len(self.ol))))
       sys.exit(2)
     if(len(self.ol[0])!=3):
-      print("ERR222: Error, figure {:s}, outline {:s}, the StartPoint_length {:d} is not 3".format(figure_id, self.outline_id, len(self.ol[0])))
+      six.print_(("ERR222: Error, figure {:s}, outline {:s}, the StartPoint_length {:d} is not 3".format(figure_id, self.outline_id, len(self.ol[0]))))
       sys.exit(2)
     for i in range(len(self.ol)):
       if((len(self.ol[i])!=3)and(len(self.ol[i])!=5)):
-        print("ERR226: Error, figure {:s}, outline {:s}, segment {:d}, length {:d} is not 3 or 5".format(figure_id, self.outline_id, i, len(self.ol[i])))
+        six.print_(("ERR226: Error, figure {:s}, outline {:s}, segment {:d}, length {:d} is not 3 or 5".format(figure_id, self.outline_id, i, len(self.ol[i]))))
         sys.exit(2)
     if((self.ol[0][0]!=self.ol[-1][-3])or(self.ol[0][1]!=self.ol[-1][-2])):
-      print("ERR182: Error, figure {:s}, outline {:s} is not closed: first point ({:0.3f}, {:0.3f}), last point ({:0.3f}, {:0.3f})".format(figure_id, self.outline_id, self.ol[0][0], self.ol[0][1], self.ol[-1][-3], self.ol[-1][-2]))
+      six.print_(("ERR182: Error, figure {:s}, outline {:s} is not closed: first point ({:0.3f}, {:0.3f}), last point ({:0.3f}, {:0.3f})".format(figure_id, self.outline_id, self.ol[0][0], self.ol[0][1], self.ol[-1][-3], self.ol[-1][-2])))
       sys.exit(2)
     if(self.ol[-1][-1]!=0):
-      print("ERR185: Error, figure {:s}, outline {:s}, last point router_bit_radius {:0.3f} is not 0.0".format(figure_id, self.outline_id, self.ol[-1][-1]))
+      six.print_(("ERR185: Error, figure {:s}, outline {:s}, last point router_bit_radius {:0.3f} is not 0.0".format(figure_id, self.outline_id, self.ol[-1][-1])))
       sys.exit(2)
     # compute statistics on the outline
     stat = {}
@@ -340,13 +341,13 @@ def check(self, figure_id=""):
         stat['y_min'] = min(stat['y_min'], self.ol[i+1][1], self.ol[i+1][3])
         stat['y_max'] = max(stat['y_max'], self.ol[i+1][1], self.ol[i+1][3])
       else:
-        print("ERR244: Error, figure_id {:s}, outline {:s}, segment {:d} unexpected length {:d}".format(figure_id, self.outline_id, i+1, len(self.ol[i+1])))
+        six.print_(("ERR244: Error, figure_id {:s}, outline {:s}, segment {:d} unexpected length {:d}".format(figure_id, self.outline_id, i+1, len(self.ol[i+1]))))
         sys.exit(2)
       if(pt1_distance<radian_epsilon):
-        print("ERR258: Error, figure_id {:s}, outline {:s}, segment {:d} pt1_distance {:0.3f} too small".format(figure_id, self.outline_id, i+1, pt1_distance))
+        six.print_(("ERR258: Error, figure_id {:s}, outline {:s}, segment {:d} pt1_distance {:0.3f} too small".format(figure_id, self.outline_id, i+1, pt1_distance)))
         sys.exit(2)
       if((pt2_distance<radian_epsilon) and not(segment_line_narc)):
-        print("ERR261: Error, figure_id {:s}, outline {:s}, segment {:d} pt2_distance {:0.3f} too small".format(figure_id, self.outline_id, i+1, pt2_distance))
+        six.print_(("ERR261: Error, figure_id {:s}, outline {:s}, segment {:d} pt2_distance {:0.3f} too small".format(figure_id, self.outline_id, i+1, pt2_distance)))
         sys.exit(2)
       # string_length
       stat['string_length_min'] = min_w_init(stat['string_length_min'], string_length)
@@ -431,11 +432,11 @@ def convert_from_old_format(self, ai_outline):
         this function should be only used during the transistion to the new format (Arc_Line_Outline object)
     """
     if(len(ai_outline)<3):
-      print("ERR378: Error, outline {:s}, convert_from_old_format failed because import list {:d} is too short".format(self.outline_id, len(ai_outline)))
+      six.print_(("ERR378: Error, outline {:s}, convert_from_old_format failed because import list {:d} is too short".format(self.outline_id, len(ai_outline))))
       sys.exit(2)
     first_segment = ai_outline[0]
     if(len(first_segment)!=3):
-      print("ERR381: Error, outline {:s}, convert_from_old_format failed because import first point length {:d} is not 3".format(self.outline_id, len(first_segment)))
+      six.print_(("ERR381: Error, outline {:s}, convert_from_old_format failed because import first point length {:d} is not 3".format(self.outline_id, len(first_segment))))
       sys.exit(2)
     self.add_StartPoint(first_segment[0], first_segment[1], rbr=first_segment[2])
     for i in range(len(ai_outline)-2):
@@ -445,21 +446,21 @@ def convert_from_old_format(self, ai_outline):
       elif(len(segment)==5):
         self.add_ArcThrTo(segment[0], segment[1], segment[2], segment[3], rbr=segment[4])
       else:
-        print("ERR390: Error, outline {:s}, convert_from_old_format failed because segment length {:d} is not 3 or 5".format(self.outline_id, len(segment)))
+        six.print_(("ERR390: Error, outline {:s}, convert_from_old_format failed because segment length {:d} is not 3 or 5".format(self.outline_id, len(segment))))
         sys.exit(2)
     last_segment = ai_outline[-1]
     if(last_segment[-1]!=0):
-      print("ERR395: Error, outline {:s}, convert_from_old_format failed because import last_point rbr {:0.3f} is not 0.0".format(self.outline_id, last_segment[-1]))
+      six.print_(("ERR395: Error, outline {:s}, convert_from_old_format failed because import last_point rbr {:0.3f} is not 0.0".format(self.outline_id, last_segment[-1])))
       sys.exit(2)
     if((last_segment[-3]!=first_segment[0])or(last_segment[-2]!=first_segment[1])):
-      print("ERR399: Error, outline {:s}, convert_from_old_format failed because import is not closed x {:0.3f} {:0.3f} y {:0.3f} {:0.3f}".format(self.outline_id, first_segment[0], last_segment[-3], first_segment[1], last_segment[-2]))
+      six.print_(("ERR399: Error, outline {:s}, convert_from_old_format failed because import is not closed x {:0.3f} {:0.3f} y {:0.3f} {:0.3f}".format(self.outline_id, first_segment[0], last_segment[-3], first_segment[1], last_segment[-2])))
       sys.exit(2)
     if(len(last_segment)==3):
       self.close_with_Line()
     elif(len(last_segment)==5):
       self.close_with_ArcThr(last_segment[0], last_segment[1])
     else:
-      print("ERR406: Error, outline {:s}, convert_from_old_format failed because last_segment length {:d} is not 3 or 5".format(self.outline_id, len(last_segment)))
+      six.print_(("ERR406: Error, outline {:s}, convert_from_old_format failed because last_segment length {:d} is not 3 or 5".format(self.outline_id, len(last_segment))))
       sys.exit(2)
 
   def cnc_cut(self):
@@ -488,7 +489,7 @@ def __init__(self, outline_id, R, CX=0.0, CY=0.0, C=()):
     """
     # check arguments
     if(R<0):
-      print("ERR151: Error, the radius R {:0.3f} is negative".format(R))
+      six.print_(("ERR151: Error, the radius R {:0.3f} is negative".format(R)))
       sys.exit(2)
     (X, Y) = get_center(CX, CY, C)
     # save values
@@ -504,7 +505,7 @@ def check(self, figure_id=""):
     radian_epsilon = math.pi/1000
     #
     if(self.radius<radian_epsilon):
-      print("ERR361: Error, figure {:s}, circle {:s} with radius {:.3f} too small".format(figure_id, self.outline_id, self.radius))
+      six.print_(("ERR361: Error, figure {:s}, circle {:s} with radius {:.3f} too small".format(figure_id, self.outline_id, self.radius)))
       sys.exit(2)
     circumference = self.radius*2*math.pi
     #
@@ -546,7 +547,7 @@ def convert_from_old_format(self, ai_outline):
         this function should be only used during the transistion to the new format (Circle_Outline object)
     """
     if(len(ai_outline)!=3):
-      print("ERR483: Error, outline {:s}, convert_from_old_format failed because import list {:d} is not 3".format(self.outline_id, len(ai_outline)))
+      six.print_(("ERR483: Error, outline {:s}, convert_from_old_format failed because import list {:d} is not 3".format(self.outline_id, len(ai_outline))))
       sys.exit(2)
     self.x = ai_outline[0]
     self.y = ai_outline[1]
@@ -606,7 +607,7 @@ def _add_outline(self, outline, hole_check=False):
     ol_stat = outline.check(self.figure_id)
     if(hole_check):
       if((self.stat['x_min']>ol_stat['x_min'])or(self.stat['x_max']<ol_stat['x_max'])or(self.stat['y_min']>ol_stat['y_min'])or(self.stat['y_max']<ol_stat['y_max'])):
-        print("ERR417: Error, figure {:s}, hole-ouline {:s} outside the boundary: x_min {:0.3f} {:0.3f}, x_max {:0.3f} {:0.3f}, y_min {:0.3f} {:0.3f}, y_max {:0.3f} {:0.3f}".format(self.figure_id, outline.outline_id, self.stat['x_min'], ol_stat['x_min'], self.stat['x_max'], ol_stat['x_max'], self.stat['y_min'], ol_stat['y_min'], self.stat['y_max'], ol_stat['y_max']))
+        six.print_(("ERR417: Error, figure {:s}, hole-ouline {:s} outside the boundary: x_min {:0.3f} {:0.3f}, x_max {:0.3f} {:0.3f}, y_min {:0.3f} {:0.3f}, y_max {:0.3f} {:0.3f}".format(self.figure_id, outline.outline_id, self.stat['x_min'], ol_stat['x_min'], self.stat['x_max'], ol_stat['x_max'], self.stat['y_min'], ol_stat['y_min'], self.stat['y_max'], ol_stat['y_max'])))
         sys.exit(2)
     self.stat['x_min'] = min_w_init(self.stat['x_min'], ol_stat['x_min'])
     self.stat['x_max'] = max_w_init(self.stat['x_max'], ol_stat['x_max'])
@@ -638,7 +639,7 @@ def _add_outline(self, outline, hole_check=False):
       self.stat['negative_rbr_max'] = max_w_init(self.stat['negative_rbr_max'], ol_stat['negative_rbr_max'])
       self.stat['negative_rbr_min'] = min_w_init(self.stat['negative_rbr_min'], ol_stat['negative_rbr_min'])
     else:
-      print("ERR429: Error, figure {:d}, outline {:s} is an invalid object".format(self.figure_id, outline.outline_id))
+      six.print_(("ERR429: Error, figure {:d}, outline {:s} is an invalid object".format(self.figure_id, outline.outline_id)))
       sys.exit(2)
 
   def add_external_outline(self, outline):
@@ -646,7 +647,7 @@ def add_external_outline(self, outline):
     """
     self.extrudable = True
     if(len(self.outlines)!=0):
-      print("ERR457: Error, figure {:s}, external_outline {:s} not added at the first position {:d}".format(self.figure_id, outline.outline_id, len(self.outlines)))
+      six.print_(("ERR457: Error, figure {:s}, external_outline {:s} not added at the first position {:d}".format(self.figure_id, outline.outline_id, len(self.outlines))))
       sys.exit(2)
     self._add_outline(outline, False)
  
@@ -654,10 +655,10 @@ def add_hole_outline(self, outline):
     """ add a hole-outline
     """
     if(len(self.outlines)==0): # check if there is already an external_outline
-      print("ERR465: Error, figure {:s}, hole_outline {:s} added before external_outline".format(self.figure_id, outline.outline_id))
+      six.print_(("ERR465: Error, figure {:s}, hole_outline {:s} added before external_outline".format(self.figure_id, outline.outline_id)))
       sys.exit(2)
     if(not self.extrudable): # check no undefine_outline have been added
-      print("ERR468: Error, figure {:s}, hole_outline {:s} added to a not extrudable figure {:d}".format(self.figure_id, outline.outline_id, len(self.outlines)))
+      six.print_(("ERR468: Error, figure {:s}, hole_outline {:s} added to a not extrudable figure {:d}".format(self.figure_id, outline.outline_id, len(self.outlines))))
       sys.exit(2)
     self._add_outline(outline, True)
 
@@ -759,7 +760,7 @@ def get_outline_index(self, ai_index):
     """ return the Outline object (Arc_Line or Circle) with the corresponding index in the outline list
     """
     if(abs(ai_index)>=len(self.outlines)):
-      print("ERR544: Error, figure {:s}, outline index {:d} is out of the range {:s}".format(self.figure_id, ai_index, len(self.outlines)))
+      six.print_(("ERR544: Error, figure {:s}, outline index {:d} is out of the range {:s}".format(self.figure_id, ai_index, len(self.outlines))))
       sys.exit(2)
     r_outline = self.outlines[ai_index]
     return(r_outline)
@@ -773,7 +774,7 @@ def get_outline_id(self, ai_outline_id):
       if(self.outlines[i].outline_id==ai_outline_id):
         idx = i
     if(idx==-1):
-      print("ERR558: Error, figure {:s}, outline_id {:s} does not exist".format(self.figure_id, ai_outline_id))
+      six.print_(("ERR558: Error, figure {:s}, outline_id {:s} does not exist".format(self.figure_id, ai_outline_id)))
       sys.exit(2)
     r_outline = self.get_outline_index(idx)
     return(r_outline)
@@ -792,7 +793,7 @@ def convert_from_old_format(self, ai_figure):
         this function should be only used during the transistion to the new format (Figure object)
     """
     if(len(ai_figure)<1):
-      print("ERR701: Error, figure {:s}, convert_from_old_format failed because import list {:d} is 0".format(self.figure_id, len(ai_figure)))
+      six.print_(("ERR701: Error, figure {:s}, convert_from_old_format failed because import list {:d} is 0".format(self.figure_id, len(ai_figure))))
       sys.exit(2)
     for i in range(len(ai_figure)):
       old_ol = ai_figure[i]
@@ -863,7 +864,7 @@ def add_figure(self, ai_figure):
     """ add a figure to the collection and update the statistics
     """
     if(not isinstance(ai_figure, Figure)):
-      print("ERR525: Error, figure_collection {:s}, add unexpected object {:s}".format(self.collection_id, ai_figure.figure_id))
+      six.print_(("ERR525: Error, figure_collection {:s}, add unexpected object {:s}".format(self.collection_id, ai_figure.figure_id)))
       sys.exit(2)
     self.figures.append(copy.deepcopy(ai_figure)) # if the reference change, the stored object doesn't change
     #self.figures.append(copy.copy(ai_figure)) # todo: check the difference between copy() and deepcopy()
@@ -922,7 +923,7 @@ def get_figure_index(self, ai_index):
     """ return the Figure object with the corresponding index in the figure list
     """
     if(abs(ai_index)>=len(self.figures)):
-      print("ERR583: Error, figure_collection {:s}, figure index {:d} is out of the range {:s}".format(self.collection_id, ai_index, len(self.figures)))
+      six.print_(("ERR583: Error, figure_collection {:s}, figure index {:d} is out of the range {:s}".format(self.collection_id, ai_index, len(self.figures))))
       sys.exit(2)
     r_figure = self.figures[ai_index]
     return(r_figure)
@@ -936,7 +937,7 @@ def get_figure_id(self, ai_figure_id):
       if(self.figures[i].figure_id==ai_figure_id):
         idx = i
     if(idx==-1):
-      print("ERR597: Error, figure_collection {:s}, figure_id {:s} does not exist".format(self.collection_id, ai_figure_id))
+      six.print_(("ERR597: Error, figure_collection {:s}, figure_id {:s} does not exist".format(self.collection_id, ai_figure_id)))
       sys.exit(2)
     r_figure = self.get_figure_index(idx)
     return(r_figure)
@@ -972,19 +973,19 @@ def convert_from_old_format(self, ai_figure_collection):
 ################################################################
 #
 # import for test only
-import outline_backends
-import design_output
+from . import outline_backends
+from . import design_output
 
 def test_c_xy():
   """ test the API function c_xy()
   """
   print("\nTest c_xy()")
   (x,y) = c_xy(5.0, 1.0, 0.0, 0.0)
-  print("c_xy 5.0, 1.0, 0, 0 : x {:0.3f} , y {:0.3f}".format(x,y))
+  six.print_(("c_xy 5.0, 1.0, 0, 0 : x {:0.3f} , y {:0.3f}".format(x,y)))
   (x,y) = c_xy(3.0, -1.0, 2.0, -1.0)
-  print("c_xy 3.0, -1.0, 2, -1.0 : x {:0.3f} , y {:0.3f}".format(x,y))
+  six.print_(("c_xy 3.0, -1.0, 2, -1.0 : x {:0.3f} , y {:0.3f}".format(x,y)))
   (x,y) = c_xy(5.0, 1.0, C=(-2,-3))
-  print("c_xy 5.0, 1.0, C=(-2,-3) : x {:0.3f} , y {:0.3f}".format(x,y))
+  six.print_(("c_xy 5.0, 1.0, C=(-2,-3) : x {:0.3f} , y {:0.3f}".format(x,y)))
 
 def test_draw_2d_1():
   """ first complete test of the draw_2d
@@ -1018,16 +1019,16 @@ def test_draw_2d_1():
   fig3.merge_figure(fig1)
   fig3.merge_figure(fig2)
   tfc.add_figure(fig3)
-  print(tfc.stat_info())
-  print(tfc.get_figure_id('first_figure').stat_info("test_figure_collection"))
-  print(tfc.get_figure_id('first_figure').get_outline_id('inner_shape').stat_info("test_figure_collection.first_figure"))
-  print(tfc.get_figure_id('first_figure').get_outline_id('inner_circle').stat_info("test_figure_collection.first_figure"))
+  six.print_((tfc.stat_info()))
+  six.print_((tfc.get_figure_id('first_figure').stat_info("test_figure_collection")))
+  six.print_((tfc.get_figure_id('first_figure').get_outline_id('inner_shape').stat_info("test_figure_collection.first_figure")))
+  six.print_((tfc.get_figure_id('first_figure').get_outline_id('inner_circle').stat_info("test_figure_collection.first_figure")))
   old_figure_collection = tfc.convert_to_old_format()
   #print("dbg904: old_figure_collection: ", old_figure_collection)
   tfc2 = Figure_Collection("test_figure_collection_reload")
   tfc2.convert_from_old_format(old_figure_collection)
-  print(tfc2.stat_info())
-  print(tfc2.get_figure_id('first_figure').stat_info("test_figure_collection_reload"))
+  six.print_((tfc2.stat_info()))
+  six.print_((tfc2.get_figure_id('first_figure').stat_info("test_figure_collection_reload")))
   b_figure = design_output.cnc_cut_figure(tfc.get_figure_id('first_figure').convert_to_old_format(), "first_figure")
   #outline_backends.figure_simple_display(b_figure, (), "test")
   outline_backends.figure_simple_display(fig1.cnc_cut(), fig1.ideal(), "test")
diff --git a/cnc25d/epicyclic_gearing.py b/cnc25d/epicyclic_gearing.py
index 7f9f35c..265c9bd 100644
--- a/cnc25d/epicyclic_gearing.py
+++ b/cnc25d/epicyclic_gearing.py
@@ -29,7 +29,8 @@
 # header for Python / FreeCAD compatibility
 ################################################################
 
-import cnc25d_api
+from . import cnc25d_api
+import six
 cnc25d_api.importing_freecad()
 
 #print("FreeCAD.Version:", FreeCAD.Version())
@@ -52,10 +53,10 @@
 #import svgwrite
 #from dxfwrite import DXFEngine
 # cnc25d
-import gearring
-import gearwheel
-import gear_profile # to get the high-level parameter to find the angle position
-import axle_lid
+from . import gearring
+from . import gearwheel
+from . import gear_profile # to get the high-level parameter to find the angle position
+from . import axle_lid
 
 ################################################################
 # epicyclic_gearing constraint_constructor
@@ -380,14 +381,14 @@ def sun_gearwheel_constraint(c):
   #tooth_check = (c['sun_gear_tooth_nb'] + c['annulus_gear_tooth_nb'])%c['planet_nb']
   tooth_check = (2*(c['sun_gear_tooth_nb'] + c['planet_gear_tooth_nb'])) % c['planet_nb']
   if(tooth_check!=0):
-    print("WARN418: Warning, tooth_check {:d} is different from 0.".format(tooth_check))
+    six.print_(("WARN418: Warning, tooth_check {:d} is different from 0.".format(tooth_check)))
     #print("tooth_check = (sun_nb {:d} + annulus_nb {:d}) % planet_nb {:d}".format(c['sun_gear_tooth_nb'], c['annulus_gear_tooth_nb'], c['planet_nb']))
-    print("tooth_check = (2*(sun_nb {:d} + planet_nb {:d})) % planet_nb {:d}".format(c['sun_gear_tooth_nb'], c['planet_gear_tooth_nb'], c['planet_nb']))
+    six.print_(("tooth_check = (2*(sun_nb {:d} + planet_nb {:d})) % planet_nb {:d}".format(c['sun_gear_tooth_nb'], c['planet_gear_tooth_nb'], c['planet_nb'])))
   for i in range(c['planet_nb']):
     a0_ai_diff = math.fmod(sun_angle_position[i]-sun_angle_position[0]+5.5*g2_pi_module_angle, g2_pi_module_angle) - 0.5*g2_pi_module_angle
     if(abs(a0_ai_diff)>radian_epsilon):
-      print("ERR414: Error, the i {:d} sun_angle_position {:0.5f} differ from the 0 sun_angle_position {:0.5f} with g2_pi_module_angle {:0.8f}".format(i, sun_angle_position[i], sun_angle_position[0], g2_pi_module_angle))
-      print("dbg417: a0_ai_diff {:0.8f}".format(a0_ai_diff))
+      six.print_(("ERR414: Error, the i {:d} sun_angle_position {:0.5f} differ from the 0 sun_angle_position {:0.5f} with g2_pi_module_angle {:0.8f}".format(i, sun_angle_position[i], sun_angle_position[0], g2_pi_module_angle)))
+      six.print_(("dbg417: a0_ai_diff {:0.8f}".format(a0_ai_diff)))
       sys.exit(2)
   gw_c['gear_initial_angle'] = sun_angle_position[0]
   return(gw_c)
@@ -419,7 +420,7 @@ def epicyclic_gearing_constraint_check(c):
     c['carrier_crenel_rbr'] = c['cnc_router_bit_radius']
   ## gearring_dedendum_to_hollow_pourcentage
   if((c['gearring_dedendum_to_hollow_pourcentage']>=100.0)or(c['gearring_dedendum_to_hollow_pourcentage']<0)):
-    print("ERR277: Error, gearring_dedendum_to_hollow_pourcentage {:0.3f} must be set between 0.0% and 100.0%".format(c['gearring_dedendum_to_hollow_pourcentage']))
+    six.print_(("ERR277: Error, gearring_dedendum_to_hollow_pourcentage {:0.3f} must be set between 0.0% and 100.0%".format(c['gearring_dedendum_to_hollow_pourcentage'])))
     sys.exit(2)
   ## tooth number
   c['annulus_gear_tooth_nb'] = c['sun_gear_tooth_nb'] + 2 * c['planet_gear_tooth_nb']
@@ -432,12 +433,12 @@ def epicyclic_gearing_constraint_check(c):
   if(c['planet_nb']==0):
     c['planet_nb'] = c['planet_number_max']
   if(c['planet_nb']>c['planet_number_max']):
-    print("ERR270: Error, planet_nb {:d} is bigger than planet_number_max {:d}".format(c['planet_nb'], c['planet_number_max']))
+    six.print_(("ERR270: Error, planet_nb {:d} is bigger than planet_number_max {:d}".format(c['planet_nb'], c['planet_number_max'])))
     sys.exit(2)
   c['epicyclic_gearing_ratio'] = float(c['sun_gear_tooth_nb'])/(c['sun_gear_tooth_nb']+c['annulus_gear_tooth_nb'])
   ## gear_addendum_dedendum_parity_slack
   if((c['gear_addendum_dedendum_parity_slack']<0)or(c['gear_addendum_dedendum_parity_slack']>30)):
-    print("ERR274: Error, gear_addendum_dedendum_parity_slack {:0.3f} is out of the range 0..30".format(c['gear_addendum_dedendum_parity_slack']))
+    six.print_(("ERR274: Error, gear_addendum_dedendum_parity_slack {:0.3f} is out of the range 0..30".format(c['gear_addendum_dedendum_parity_slack'])))
     sys.exit(2)
   c['addendum_dedendum_parity'] = 50.0-c['gear_addendum_dedendum_parity_slack']/2.0
   c['first_planet_position_angle'] = c['planet_carrier_angle']
@@ -502,23 +503,23 @@ def epicyclic_gearing_constraint_check(c):
   #  print("ERR443: Error, carrier_central_hole_diameter {:0.3f} is bigger than carrier_central_diameter {:0.3f}".format(c['carrier_central_hole_diameter'], c['carrier_central_diameter']))
   #  sys.exit(2)
   if(c['carrier_leg_hole_radius']>c['carrier_leg_radius']):
-    print("ERR446: Error, carrier_leg_hole_radius {:0.3f} is bigger than carrier_leg_radius {:0.3f}".format(c['carrier_leg_hole_radius'], c['carrier_leg_radius']))
+    six.print_(("ERR446: Error, carrier_leg_hole_radius {:0.3f} is bigger than carrier_leg_radius {:0.3f}".format(c['carrier_leg_hole_radius'], c['carrier_leg_radius'])))
     sys.exit(2)
   if(c['carrier_peripheral_internal_radius']>c['sun_planet_length']):
-    print("ERR448: Error, carrier_peripheral_internal_radius {:0.3f} is bigger than sun_planet_length {:0.3f}".format(c['carrier_peripheral_internal_radius'], c['sun_planet_length']))
+    six.print_(("ERR448: Error, carrier_peripheral_internal_radius {:0.3f} is bigger than sun_planet_length {:0.3f}".format(c['carrier_peripheral_internal_radius'], c['sun_planet_length'])))
     sys.exit(2)
   if(c['carrier_peripheral_internal_radius']<(c['carrier_central_radius']+3*c['carrier_sr'])):
     #print("WARN455: Warning, carrier_peripheral_internal_radius {:0.3f} is too small compare to  carrier_central_radius {:0.3f} and carrier_smoothing_radius {:0.3f}".format(c['carrier_peripheral_internal_radius'], c['carrier_central_radius'], c['carrier_sr']))
     c['carrier_hollow_disable'] = True
   if(c['carrier_peripheral_external_radius']<(c['sun_planet_length']+c['carrier_leg_radius'])):
-    print("WARN461: Warning, carrier_peripheral_external_radius {:0.3f} is too small compare to sun_planet_length {:0.3f} and carrier_leg_radius {:0.3f}".format(c['carrier_peripheral_external_radius'], c['sun_planet_length'], c['carrier_leg_radius']))
+    six.print_(("WARN461: Warning, carrier_peripheral_external_radius {:0.3f} is too small compare to sun_planet_length {:0.3f} and carrier_leg_radius {:0.3f}".format(c['carrier_peripheral_external_radius'], c['sun_planet_length'], c['carrier_leg_radius'])))
     sys.exit(2)
   c['carrier_crenel'] = True
   if(c['carrier_crenel_height']==0):
     c['carrier_crenel'] = False
   if(c['carrier_crenel']):
     if(c['carrier_crenel_width']<7.0*c['carrier_crenel_rbr']):
-      print("ERR468: Error, carrier_crenel_width {:0.3f} is too small compare to carrier_crenel_router_bit_radius {:0.3f}".format(c['carrier_crenel_width'], c['carrier_crenel_rbr']))
+      six.print_(("ERR468: Error, carrier_crenel_width {:0.3f} is too small compare to carrier_crenel_router_bit_radius {:0.3f}".format(c['carrier_crenel_width'], c['carrier_crenel_rbr'])))
       sys.exit(2)
     if(c['carrier_crenel_height']<3*c['carrier_crenel_rbr']):
       c['carrier_crenel_type'] = 2
@@ -527,48 +528,48 @@ def epicyclic_gearing_constraint_check(c):
   #print("dbg509: carrier_crenel_router_bit_radius {:0.3f}  carrier_crenel_type {:d}".format(c['carrier_crenel_rbr'], c['carrier_crenel_type']))
   if(c['carrier_hole_radius']>0):
     if(c['carrier_hole_position_radius']<(c['sun_planet_length']+c['carrier_leg_hole_radius']+c['carrier_hole_radius']+radian_epsilon)):
-      print("ERR544: Error, carrier_hole_position_radius {:0.3f} is too small compare to sun_planet_length {:0.3f}, carrier_leg_hole_radius {:0.3f} and carrier_hole_radius {:0.3f}".format(c['carrier_hole_position_radius'], c['sun_planet_length'], c['carrier_leg_hole_radius'], c['carrier_hole_radius']))
+      six.print_(("ERR544: Error, carrier_hole_position_radius {:0.3f} is too small compare to sun_planet_length {:0.3f}, carrier_leg_hole_radius {:0.3f} and carrier_hole_radius {:0.3f}".format(c['carrier_hole_position_radius'], c['sun_planet_length'], c['carrier_leg_hole_radius'], c['carrier_hole_radius'])))
       sys.exit(2)
     if(c['carrier_hole_position_radius']>(c['carrier_peripheral_external_radius']-c['carrier_crenel_height']-c['carrier_hole_radius']-radian_epsilon)):
-      print("ERR548: Error, carrier_hole_position_radius {:0.3f} is too big compare to carrier_peripheral_external_radius {:0.3f} carrier_crenel_height {:0.3f} and carrier_hole_radius {:0.3f}".format(c['carrier_hole_position_radius'], c['carrier_peripheral_external_radius'], c['carrier_crenel_height'], c['carrier_hole_radius']))
+      six.print_(("ERR548: Error, carrier_hole_position_radius {:0.3f} is too big compare to carrier_peripheral_external_radius {:0.3f} carrier_crenel_height {:0.3f} and carrier_hole_radius {:0.3f}".format(c['carrier_hole_position_radius'], c['carrier_peripheral_external_radius'], c['carrier_crenel_height'], c['carrier_hole_radius'])))
       sys.exit(2)
   if(c['carrier_double_hole_length']<0):
-    print("ERR658: Error, carrier_double_hole_length {:0.3f} should be positive".format(c['carrier_double_hole_length']))
+    six.print_(("ERR658: Error, carrier_double_hole_length {:0.3f} should be positive".format(c['carrier_double_hole_length'])))
     sys.exit(2)
   elif(c['carrier_double_hole_length']>0):
     if(c['carrier_hole_radius']==0):
-      print("ERR662: Error, carrier_double_hole_length {:0.3f} is positive whereas carrier_hole_radius is set to zero".format(c['carrier_double_hole_length']))
+      six.print_(("ERR662: Error, carrier_double_hole_length {:0.3f} is positive whereas carrier_hole_radius is set to zero".format(c['carrier_double_hole_length'])))
       sys.exit(2)
   ### top (axle-lid)
   c['top_clearance_radius'] = c['top_clearance_diameter']/2.0
   if(c['top_clearance_radius'] == 0):
     c['top_clearance_radius'] = (c['carrier_peripheral_external_radius'] + c['holder_radius'])/2.0 # default value
   if(c['top_clearance_radius'] < c['carrier_peripheral_external_radius']):
-    print("ERR968: Error, top_clearance_radius {:0.3f} is smaller than carrier_peripheral_external_radius {:0.3f}".format(c['top_clearance_radius'], c['carrier_peripheral_external_radius']))
+    six.print_(("ERR968: Error, top_clearance_radius {:0.3f} is smaller than carrier_peripheral_external_radius {:0.3f}".format(c['top_clearance_radius'], c['carrier_peripheral_external_radius'])))
     sys.exit(2)
   if(c['top_clearance_radius'] > c['holder_radius']):
-    print("ERR971: Error, top_clearance_radius {:0.3f} is bigger than holder_radius {:0.3f}".format(c['top_clearance_radius'], c['holder_radius']))
+    six.print_(("ERR971: Error, top_clearance_radius {:0.3f} is bigger than holder_radius {:0.3f}".format(c['top_clearance_radius'], c['holder_radius'])))
     sys.exit(2)
   c['top_axle_hole_radius'] = c['top_axle_hole_diameter']/2.0
   if(c['top_axle_hole_radius'] == 0):
     if(c['sun_axle_type'] != 'circle'):
-      print("WARN978: Warning, sun_axle_type {:s} is not set to circle but sun_axle_x_width {:0.3f} is used to set the top_axle_hole_diameter".format(c['sun_axle_type'], c['sun_axle_x_width']))
+      six.print_(("WARN978: Warning, sun_axle_type {:s} is not set to circle but sun_axle_x_width {:0.3f} is used to set the top_axle_hole_diameter".format(c['sun_axle_type'], c['sun_axle_x_width'])))
     c['top_axle_hole_radius'] = c['sun_axle_x_width']/2.0
   c['top_central_radius'] = c['top_central_diameter']/2.0
   if(c['top_central_radius'] == 0):
     c['top_central_radius'] = 3*c['top_axle_hole_radius']
   if(c['top_central_radius']<c['top_axle_hole_radius']):
-    print("ERR984: Error, top_central_radius {:0.3f} is smaller than top_axle_hole_radius {:0.3f}".format(c['top_central_radius'], c['top_axle_hole_radius']))
+    six.print_(("ERR984: Error, top_central_radius {:0.3f} is smaller than top_axle_hole_radius {:0.3f}".format(c['top_central_radius'], c['top_axle_hole_radius'])))
     sys.exit(2)
   if(c['top_central_radius']>c['holder_radius']):
-    print("ERR987: Error, top_central_radius {:0.3f} is bigger than holder_radius {:0.3f}".format(c['top_central_radius'], c['holder_radius']))
+    six.print_(("ERR987: Error, top_central_radius {:0.3f} is bigger than holder_radius {:0.3f}".format(c['top_central_radius'], c['holder_radius'])))
     sys.exit(2)
   ### inout_in_hole_diameter
   #c['input_axle_shaft_radius'] = (c['input_gearwheel_tooth_nb']+2)*c['input_gearwheel_module']/2.0 # no, to avoid Z-axis issue, we don't want to make this radius as small as possible, but set it to carrier_peripheral_external_radius. Nice to have: input_axle_shaft_radius < (c['annulus_gear_tooth_nb']-2)*gear_module
   #c['input_axle_shaft_radius'] = c['carrier_peripheral_external_radius']
   c['input_axle_shaft_radius'] = max((c['input_gearwheel_tooth_nb']+2)*c['input_gearwheel_module']/2.0, c['carrier_peripheral_external_radius'])
   if(c['input_axle_shaft_radius']>(c['annulus_gear_tooth_nb']-2)*c['gear_module']/2.0-radian_epsilon):
-    print("WARN910: Warning, input_axle_shaft_radius {:0.3f} is too big compare to annulus_gear_tooth_nb {:d} and gear_module {:0.3f}".format(c['input_axle_shaft_radius'], c['annulus_gear_tooth_nb'], c['gear_module']))
+    six.print_(("WARN910: Warning, input_axle_shaft_radius {:0.3f} is too big compare to annulus_gear_tooth_nb {:d} and gear_module {:0.3f}".format(c['input_axle_shaft_radius'], c['annulus_gear_tooth_nb'], c['gear_module'])))
   c['inout_in_hole_diameter'] = 2*(c['input_axle_shaft_radius']+c['input_cover_extra_space'])
   ### inout_out_hole_diameter
   #output_axle_shaft_radius = (c['output_gearwheel_tooth_nb']+2)*c['output_gearwheel_module']/2.0 # no, it's not defined by the output gearwheel
@@ -627,7 +628,7 @@ def carrier_crenel_outline(nai_radius):
       carrier_peripheral_portion_angle = 2*math.pi/(2*c['planet_nb'])
       carrier_peripheral_arc_half_angle = (carrier_peripheral_portion_angle - 2 * crenel_width_half_angle)/2.0
       if(carrier_peripheral_arc_half_angle<radian_epsilon):
-        print("ERR493: Error, carrier_peripheral_arc_half_angle {:0.3f} is negative or too small".format(carrier_peripheral_arc_half_angle))
+        six.print_(("ERR493: Error, carrier_peripheral_arc_half_angle {:0.3f} is negative or too small".format(carrier_peripheral_arc_half_angle)))
         sys.exit(2)
       cp_A = [(0.0+cpe_radius*math.cos(-1*crenel_width_half_angle), 0.0+cpe_radius*math.sin(-1*crenel_width_half_angle), 0)]
       arc_middle_a = crenel_width_half_angle + carrier_peripheral_arc_half_angle
@@ -663,7 +664,7 @@ def carrier_crenel_outline(nai_radius):
       (crenel_A, crenel_width_half_angle) = carrier_crenel_outline(AIl)
       arc_half_angle = (math.pi/2 - LAOa - crenel_width_half_angle)/2.0
       if(arc_half_angle<radian_epsilon):
-        print("ERR596: Error, arc_half_angle {:0.3f} is negative or too small".format(arc_half_angle))
+        six.print_(("ERR596: Error, arc_half_angle {:0.3f} is negative or too small".format(arc_half_angle)))
         sys.exit(2)
       leg_A.append((OAl+AIl*math.cos(ta1+1*arc_half_angle), AIl*math.sin(ta1+1*arc_half_angle), OAl+AIl*math.cos(ta1+2*arc_half_angle), AIl*math.sin(ta1+2*arc_half_angle), 0))
       leg_A.extend(cnc25d_api.outline_shift_x(crenel_A, OAl, 1))
@@ -724,7 +725,7 @@ def carrier_crenel_outline(nai_radius):
       cia_half = cia
       arc_half = (leg_hole_portion - 2*cia_half)/2.0
       if(arc_half<radian_epsilon):
-        print("ERR551: Error, rear planet-carrier arc_half {:0.3f} is negative or too small".format(arc_half))
+        six.print_(("ERR551: Error, rear planet-carrier arc_half {:0.3f} is negative or too small".format(arc_half)))
         sys.exit(2)
       cpi_A = [(0.0+cpi_radius*math.cos(-1*cia_half), 0.0+cpi_radius*math.sin(-1*cia_half), c['carrier_sr'])]
       short_radius = c['sun_planet_length'] - cl_radius
@@ -750,11 +751,11 @@ def carrier_crenel_outline(nai_radius):
       middle_radius_intersection = True
     # intersection carrier_leg_middle_radius and carrier_peripheral_external_radius from planet
     if(c['sun_planet_length']+c['carrier_leg_middle_radius']<c['carrier_peripheral_external_radius']):
-      print("ERR731: Error, carrier_leg_middle_radius {:0.3f} is too small compare to sun_planet_length {:0.3f} and carrier_peripheral_external_radius {:0.3f}".format(c['carrier_leg_middle_radius'], c['sun_planet_length'], c['carrier_peripheral_external_radius']))
+      six.print_(("ERR731: Error, carrier_leg_middle_radius {:0.3f} is too small compare to sun_planet_length {:0.3f} and carrier_peripheral_external_radius {:0.3f}".format(c['carrier_leg_middle_radius'], c['sun_planet_length'], c['carrier_peripheral_external_radius'])))
       sys.exit(2)
     cos_imea = (c['carrier_leg_middle_radius']**2+c['sun_planet_length']**2-c['carrier_peripheral_external_radius']**2)/(2*c['carrier_leg_middle_radius']*c['sun_planet_length'])
     if((cos_imea<-1)or(cos_imea>1)):
-      print("ERR730: Error, cos_imea {:0.3f} is out of the range -1,1".format(cos_imea))
+      six.print_(("ERR730: Error, cos_imea {:0.3f} is out of the range -1,1".format(cos_imea)))
       sys.exit(2)
     imea = math.acos(cos_imea)
     # intersection carrier_leg_middle_radius and carrier_peripheral_internal_radius from planet
@@ -767,7 +768,7 @@ def carrier_crenel_outline(nai_radius):
         imia = imia_0 + math.acos((planet_planet_length**2+0*c['carrier_leg_middle_radius']**2)/(2*planet_planet_length*c['carrier_leg_middle_radius']))
     pla = (imea-imia)/2.0
     if(pla<radian_epsilon):
-      print("ERR628: Error, imia {:0.3f} is larger than imea {:0.3f}".format(imia, imea))
+      six.print_(("ERR628: Error, imia {:0.3f} is larger than imea {:0.3f}".format(imia, imea)))
       sys.exit(2)
     # intersection carrier_leg_middle_radius and carrier_peripheral_external_radius from sun
     imefsa = math.acos((c['sun_planet_length']**2+c['carrier_peripheral_external_radius']**2-c['carrier_leg_middle_radius']**2)/(2*c['sun_planet_length']*c['carrier_peripheral_external_radius']))
@@ -784,7 +785,7 @@ def carrier_crenel_outline(nai_radius):
       (crenel_A, crenel_width_half_angle) = carrier_crenel_outline(cpe_radius)
       carrier_peripheral_arc_half_angle = (leg_hole_portion - 2 * imefsa - 2 * crenel_width_half_angle)/4.0
       if(carrier_peripheral_arc_half_angle<radian_epsilon):
-        print("ERR635: Error, middle carrier_peripheral_arc_half_angle {:0.3f} is negative or too small".format(carrier_peripheral_arc_half_angle))
+        six.print_(("ERR635: Error, middle carrier_peripheral_arc_half_angle {:0.3f} is negative or too small".format(carrier_peripheral_arc_half_angle)))
         sys.exit(2)
       arc_middle_a = crenel_width_half_angle + carrier_peripheral_arc_half_angle
       arc_end_a = arc_middle_a + carrier_peripheral_arc_half_angle
@@ -915,7 +916,7 @@ def carrier_crenel_outline(nai_radius):
       output_axle_shaft_figure.extend(output_gearwheel_figure[1:])
     inout_out_axle_shaft_figure = output_axle_shaft_figure[:] # inherit outline and holes from output_axle_shaft_figure
     if(c['sun_axle_type']!='circle'):
-      print("WARN945: Warning, sun_axle_type {:s} is not circle but sun_axle_x_width {:0.3f} is used as axle diameter for the output shaft".format(c['sun_axle_type'], c['sun_axle_x_width']))
+      six.print_(("WARN945: Warning, sun_axle_type {:s} is not circle but sun_axle_x_width {:0.3f} is used as axle diameter for the output shaft".format(c['sun_axle_type'], c['sun_axle_x_width'])))
     output_axle_shaft_figure.append((0.0, 0.0, c['sun_axle_x_width']/2.0))
     output_cover_shaft_merge_figure.extend(output_cover_figure) # output_cover_shaft_merge_figure
     output_cover_shaft_merge_figure.extend(output_axle_shaft_figure)
diff --git a/cnc25d/export_2d.py b/cnc25d/export_2d.py
index e4d1e65..f314579 100644
--- a/cnc25d/export_2d.py
+++ b/cnc25d/export_2d.py
@@ -28,7 +28,7 @@
 # header for Python / FreeCAD compatibility
 ################################################################
 
-import importing_freecad
+from . import importing_freecad
 importing_freecad.importing_freecad()
 
 #print("FreeCAD.Version:", FreeCAD.Version())
diff --git a/cnc25d/gear_profile.py b/cnc25d/gear_profile.py
index fbc6003..417ba27 100644
--- a/cnc25d/gear_profile.py
+++ b/cnc25d/gear_profile.py
@@ -30,7 +30,8 @@
 # header for Python / FreeCAD compatibility
 ################################################################
 
-import cnc25d_api
+from . import cnc25d_api
+import six
 cnc25d_api.importing_freecad()
 
 #print("FreeCAD.Version:", FreeCAD.Version())
@@ -46,7 +47,7 @@
 #from datetime import datetime
 #import os, errno
 #import re
-import Tkinter # to display the outline in a small GUI
+from six.moves import tkinter # to display the outline in a small GUI
 import time # for time.sleep to help Tkinter to finish properly
 # FreeCAD
 import Part
@@ -55,7 +56,7 @@
 #import svgwrite
 #from dxfwrite import DXFEngine
 # cnc25d
-from gear_profile_outline import *
+from .gear_profile_outline import *
 
 ################################################################
 # module variable
@@ -269,20 +270,20 @@ def gear_profile_constraint_check(c):
   if((c['gear_type']=='e')or(c['gear_type']=='i')or(c['gear_type']=='l')):
     g1_type = c['gear_type']
   else:
-    print("ERR111: Error, the gear_type {:s} is not valid!".format(c['gear_type']))
+    six.print_(("ERR111: Error, the gear_type {:s} is not valid!".format(c['gear_type'])))
     sys.exit(2)
   # g2_type
   if((c['second_gear_type']=='e')or(c['second_gear_type']=='i')or(c['second_gear_type']=='l')):
     g2_type = c['second_gear_type']
   else:
-    print("ERR511: Error, the gear_type {:s} is not valid!".format(c['second_gear_type']))
+    six.print_(("ERR511: Error, the gear_type {:s} is not valid!".format(c['second_gear_type'])))
     sys.exit(2)
   # check of the type cross compatibility
   if((g1_type=='i')and(g2_type!='e')):
-    print("ERR512: Error, internal gear is only compatible with external gear. g1_type: {:s}  g2_type: {:s}".format(g1_type, g2_type))
+    six.print_(("ERR512: Error, internal gear is only compatible with external gear. g1_type: {:s}  g2_type: {:s}".format(g1_type, g2_type)))
     sys.exit(2)
   if((g1_type=='l')and(g2_type!='e')):
-    print("ERR512: Error, linear gear is only compatible with external gear. g1_type: {:s}  g2_type: {:s}".format(g1_type, g2_type))
+    six.print_(("ERR512: Error, linear gear is only compatible with external gear. g1_type: {:s}  g2_type: {:s}".format(g1_type, g2_type)))
     sys.exit(2)
   g1_param['gear_type'] = g1_type
   g2_param['gear_type'] = g2_type
@@ -292,14 +293,14 @@ def gear_profile_constraint_check(c):
     print("ERR112: Error, the gear_tooth_nb must be set!")
     sys.exit(2)
   if(g1_n<3):
-    print("ERR113: Error, the gear_tooth_nb {:d} must be equal or bigger than 3!".format(g1_n))
+    six.print_(("ERR113: Error, the gear_tooth_nb {:d} must be equal or bigger than 3!".format(g1_n)))
     sys.exit(2)
   g2_n = c['second_gear_tooth_nb']
   g2_exist = False
   if(g2_n!=0):
     g2_exist = True
   if(g2_exist and (g1_n<3)):
-    print("ERR114: Error, the second_gear_tooth_nb {:d} must be equal or bigger than 3!".format(g2_n))
+    six.print_(("ERR114: Error, the second_gear_tooth_nb {:d} must be equal or bigger than 3!".format(g2_n)))
     sys.exit(2)
   #g1_param['gear_exist'] = True
   #g2_param['gear_exist'] = g2_exist
@@ -314,7 +315,7 @@ def gear_profile_constraint_check(c):
     g1_m_set = True
   if(c['gear_primitive_diameter']>0):
     if(g1_m_set):
-      print("ERR115: Error, too much constraints! the gear_module is already set to {:0.2f}!".format(g1_m))
+      six.print_(("ERR115: Error, too much constraints! the gear_module is already set to {:0.2f}!".format(g1_m)))
       sys.exit(2)
     else:
       if((g1_type=='i')or(g1_type=='e')):
@@ -327,7 +328,7 @@ def gear_profile_constraint_check(c):
       print("ERR116: Error, set second_gear_tooth_nb to use second_gear_primitive_diameter")
       sys.exit(2)
     elif(g1_m_set):
-      print("ERR117: Error, too much constraints! the gear_module is already set to {:0.2f}!".format(g1_m))
+      six.print_(("ERR117: Error, too much constraints! the gear_module is already set to {:0.2f}!".format(g1_m)))
       sys.exit(2)
     else:
       if((g2_type=='i')or(g2_type=='e')):
@@ -360,7 +361,7 @@ def gear_profile_constraint_check(c):
   # addendum_dedendum_parity
   g1_adp = float(c['gear_addendum_dedendum_parity'])/100
   if((g1_adp<=0)or(g1_adp>=1)):
-    print("ERR118: Error, the gear_addendum_dedendum_parity {:0.2f} must be set strictly between 0% and 100%!".format(c['gear_addendum_dedendum_parity']))
+    six.print_(("ERR118: Error, the gear_addendum_dedendum_parity {:0.2f} must be set strictly between 0% and 100%!".format(c['gear_addendum_dedendum_parity'])))
     sys.exit(2)
   g2_adp = 1-g1_adp
   if(c['second_gear_addendum_dedendum_parity']>0):
@@ -371,7 +372,7 @@ def gear_profile_constraint_check(c):
       #print("WARN211: Warning, second_gear_addendum_dedendum_parity is used for irregular cases.")
       g2_adp = float(c['second_gear_addendum_dedendum_parity'])/100
   if((g2_adp<=0)or(g2_adp>=1)):
-    print("ERR119: Error, the second_gear_addendum_dedendum_parity {:0.2f} must be set strictly between 0% and 100%!".format(c['second_gear_addendum_dedendum_parity']))
+    six.print_(("ERR119: Error, the second_gear_addendum_dedendum_parity {:0.2f} must be set strictly between 0% and 100%!".format(c['second_gear_addendum_dedendum_parity'])))
     sys.exit(2)
   g1_param['addendum_dedendum_parity'] = g1_adp
   g2_param['addendum_dedendum_parity'] = g2_adp
@@ -420,11 +421,11 @@ def gear_profile_constraint_check(c):
   g2_small_r = min(g2_ar, g2_dr) # small radius
   g2_big_r = max(g2_ar, g2_dr) # big radius
   if(g1_a_delta>aal+g2_d_delta):
-    print("WARN213: Warning, the addendum {:0.2f} of the first gear is too big, other the dedendum {:0.2f} of the other gear is too small (second_gear_additional_axis_length={:0.2f})!".format(g1_a_delta, g2_d_delta, aal))
+    six.print_(("WARN213: Warning, the addendum {:0.2f} of the first gear is too big, other the dedendum {:0.2f} of the other gear is too small (second_gear_additional_axis_length={:0.2f})!".format(g1_a_delta, g2_d_delta, aal)))
   if(g2_a_delta>aal+g1_d_delta):
-    print("WARN214: Warning, the addendum {:0.2f} of the second gear is too big, other the dedendum {:0.2f} of the other gear is too small (second_gear_additional_axis_length={:0.2f})!".format(g2_a_delta, g1_d_delta, aal))
+    six.print_(("WARN214: Warning, the addendum {:0.2f} of the second gear is too big, other the dedendum {:0.2f} of the other gear is too small (second_gear_additional_axis_length={:0.2f})!".format(g2_a_delta, g1_d_delta, aal)))
   if(g1_a_delta+g2_a_delta<aal):
-    print("WARN215: Warning, the (second_gear_additional_axis_length {:0.2f} is too big compare to the addendum {:0.2f} and {:0.2f}!".format(aal, g1_a_delta, g2_a_delta))
+    six.print_(("WARN215: Warning, the (second_gear_additional_axis_length {:0.2f} is too big compare to the addendum {:0.2f} and {:0.2f}!".format(aal, g1_a_delta, g2_a_delta)))
   g1_param['tooth_half_height'] = g1_thh # g1
   g1_param['addendum_height']   = g1_a_delta
   g1_param['dedendum_height']   = g1_d_delta
@@ -460,10 +461,10 @@ def gear_profile_constraint_check(c):
         print("ERR121: Error, set second_gear_tooth_nb to use second_gear_base_diameter")
         sys.exit(2)
       elif(g2_type=='l'):
-        print("ERR921: Error, the second gear type {:s} is a gearbar".format(g2_type))
+        six.print_(("ERR921: Error, the second gear type {:s} is a gearbar".format(g2_type)))
         sys.exit(2)
       elif(g1_brp_set):
-        print("ERR122: Error, too much constraints! gear_base_diameter is already set to {:0.2f}".format(g1_brp*2))
+        six.print_(("ERR122: Error, too much constraints! gear_base_diameter is already set to {:0.2f}".format(g1_brp*2)))
         sys.exit(2)
       else:
         g1_brp = float(c['second_gear_base_diameter']*g1_n)/(2*g2_n)
@@ -473,17 +474,17 @@ def gear_profile_constraint_check(c):
         print("ERR811: Error, set second_gear_tooth_nb to use gearbar_slope")
         sys.exit(2)
       elif((g2_type=='e')or(g2_type=='i')):
-        print("ERR812: Error, the second gear type {:s} is not a gearbar".format(g2_type))
+        six.print_(("ERR812: Error, the second gear type {:s} is not a gearbar".format(g2_type)))
         sys.exit(2)
       elif(g1_brp_set):
-        print("ERR813: Error, too much constraints! gear_base_diameter is already set to {:0.2f}".format(g1_brp*2))
+        six.print_(("ERR813: Error, too much constraints! gear_base_diameter is already set to {:0.2f}".format(g1_brp*2)))
         sys.exit(2)
       else:
         g1_brp = g1_pr*math.cos(c['gearbar_slope'])
         g1_brp_set = True
     if(c['gear_force_angle']>0):
       if(g1_brp_set):
-        print("ERR123: Error, too much constraints! gear_base_diameter is already set to {:0.2f}".format(g1_brp*2))
+        six.print_(("ERR123: Error, too much constraints! gear_base_diameter is already set to {:0.2f}".format(g1_brp*2)))
         sys.exit(2)
       else:
         g1_brp = g1_pr*math.cos(c['gear_force_angle'])
@@ -499,10 +500,10 @@ def gear_profile_constraint_check(c):
         print("ERR121: Error, set second_gear_tooth_nb to use second_gear_base_diameter_n")
         sys.exit(2)
       elif(g2_type=='l'):
-        print("ERR922: Error, the second gear type {:s} is a gearbar".format(g2_type))
+        six.print_(("ERR922: Error, the second gear type {:s} is a gearbar".format(g2_type)))
         sys.exit(2)
       elif(g1_brn_set):
-        print("ERR122: Error, too much constraints! gear_base_diameter_n is already set to {:0.2f}".format(g1_brn*2))
+        six.print_(("ERR122: Error, too much constraints! gear_base_diameter_n is already set to {:0.2f}".format(g1_brn*2)))
         sys.exit(2)
       else:
         g1_brn = float(c['second_gear_base_diameter_n']*g1_n)/(2*g2_n)
@@ -512,17 +513,17 @@ def gear_profile_constraint_check(c):
         print("ERR821: Error, set second_gear_tooth_nb to use gearbar_slope_n")
         sys.exit(2)
       elif((g2_type=='e')or(g2_type=='i')):
-        print("ERR822: Error, the second gear type {:s} is not a gearbar".format(g2_type))
+        six.print_(("ERR822: Error, the second gear type {:s} is not a gearbar".format(g2_type)))
         sys.exit(2)
       elif(g1_brn_set):
-        print("ERR823: Error, too much constraints! gear_base_diameter is already set to {:0.2f}".format(g1_brn*2))
+        six.print_(("ERR823: Error, too much constraints! gear_base_diameter is already set to {:0.2f}".format(g1_brn*2)))
         sys.exit(2)
       else:
         g1_brn = g1_pr*math.cos(c['gearbar_slope_n'])
         g1_brn_set = True
     if(c['gear_force_angle_n']>0):
       if(g1_brn_set):
-        print("ERR123: Error, too much constraints! gear_base_diameter is already set to {:0.2f}".format(g1_brn*2))
+        six.print_(("ERR123: Error, too much constraints! gear_base_diameter is already set to {:0.2f}".format(g1_brn*2)))
         sys.exit(2)
       else:
         g1_brn = g1_pr*math.cos(c['gear_force_angle_n'])
@@ -530,7 +531,7 @@ def gear_profile_constraint_check(c):
   elif(g1_type=='l'):
     g1_sp_set = False
     if((c['gear_base_diameter']>0)or(c['gear_base_diameter_n']>0)):
-      print("ERR331: Error, for gearbar use gearbar_slope and gearbar_slope_n instead of gear_base_diameter {:0.3f}  and gear_base_diameter_n {:0.3f}".format(c['gear_base_diameter'], c['gear_base_diameter_n']))
+      six.print_(("ERR331: Error, for gearbar use gearbar_slope and gearbar_slope_n instead of gear_base_diameter {:0.3f}  and gear_base_diameter_n {:0.3f}".format(c['gear_base_diameter'], c['gear_base_diameter_n'])))
       sys.exit(2)
     if(c['gearbar_slope']>0):
       g1_sp = c['gearbar_slope']
@@ -544,7 +545,7 @@ def gear_profile_constraint_check(c):
         print("ERR521: Error, set second_gear_tooth_nb to use second_gear_base_diameter")
         sys.exit(2)
       elif(g1_sp_set):
-        print("ERR523: Error, too much constraints! gearbar_slope_angle is already set to {:0.2f}".format(g1_sp))
+        six.print_(("ERR523: Error, too much constraints! gearbar_slope_angle is already set to {:0.2f}".format(g1_sp)))
         sys.exit(2)
       else:
         g1_sp = math.acos(float(c['second_gear_base_diameter'])/(2*g2_pr))
@@ -566,7 +567,7 @@ def gear_profile_constraint_check(c):
         print("ERR531: Error, set second_gear_tooth_nb to use second_gear_base_diameter_n")
         sys.exit(2)
       elif(g1_sn_set):
-        print("ERR532: Error, too much constraints! gearbar_slope_angle_n is already set to {:0.2f}".format(g1_sp))
+        six.print_(("ERR532: Error, too much constraints! gearbar_slope_angle_n is already set to {:0.2f}".format(g1_sp)))
         sys.exit(2)
       else:
         g1_sn = math.acos(float(c['second_gear_base_diameter_n'])/(2*g2_pr))
@@ -589,27 +590,27 @@ def gear_profile_constraint_check(c):
   # base radius check
   if((g1_type=='e')or(g1_type=='i')):
     if(g1_brp>g1_small_r):
-      print("WARN216: Warning, g1_brp {:0.2f} is bigger than g1_small_r {:0.2f}".format(g1_brp, g1_small_r))
+      six.print_(("WARN216: Warning, g1_brp {:0.2f} is bigger than g1_small_r {:0.2f}".format(g1_brp, g1_small_r)))
     if(g1_brn>g1_small_r):
-      print("WARN246: Warning, g1_brn {:0.2f} is bigger than g1_small_r {:0.2f}".format(g1_brn, g1_small_r))
+      six.print_(("WARN246: Warning, g1_brn {:0.2f} is bigger than g1_small_r {:0.2f}".format(g1_brn, g1_small_r)))
     if(g1_brp!=g1_brn):
-      print("WARN218: Warning, g1_brp {:0.2f} and g1_brn {:0.2f} are different. The gear_tooth are asymmetrical!".format(g1_brp, g1_brn))
+      six.print_(("WARN218: Warning, g1_brp {:0.2f} and g1_brn {:0.2f} are different. The gear_tooth are asymmetrical!".format(g1_brp, g1_brn)))
     if(g1_brp>g1_big_r):
-      print("ERR616: Error, g1_brp {:0.2f} is bigger than g1_big_r {:0.2f}".format(g1_brp, g1_big_r))
+      six.print_(("ERR616: Error, g1_brp {:0.2f} is bigger than g1_big_r {:0.2f}".format(g1_brp, g1_big_r)))
       sys.exit(2)
     if(g1_brn>g1_big_r):
-      print("ERR646: Error, g1_brn {:0.2f} is bigger than g1_big_r {:0.2f}".format(g1_brn, g1_big_r))
+      six.print_(("ERR646: Error, g1_brn {:0.2f} is bigger than g1_big_r {:0.2f}".format(g1_brn, g1_big_r)))
       sys.exit(2)
   if((g2_type=='e')or(g2_type=='i')):
     if(g2_exist and (g2_brp>g2_small_r)):
-      print("WARN217: Warning, g2_brp {:0.2f} is bigger than g2_small_r {:0.2f}".format(g2_brp, g2_small_r))
+      six.print_(("WARN217: Warning, g2_brp {:0.2f} is bigger than g2_small_r {:0.2f}".format(g2_brp, g2_small_r)))
     if(g2_exist and (g2_brn>g2_small_r)):
-      print("WARN247: Warning, g2_brn {:0.2f} is bigger than g2_small_r {:0.2f}".format(g2_brn, g2_small_r))
+      six.print_(("WARN247: Warning, g2_brn {:0.2f} is bigger than g2_small_r {:0.2f}".format(g2_brn, g2_small_r)))
     if(g2_exist and (g2_brp>g2_big_r)):
-      print("ERR617: Error, g2_brp {:0.2f} is bigger than g2_big_r {:0.2f}".format(g2_brp, g2_big_r))
+      six.print_(("ERR617: Error, g2_brp {:0.2f} is bigger than g2_big_r {:0.2f}".format(g2_brp, g2_big_r)))
       sys.exit(2)
     if(g2_exist and (g2_brn>g2_big_r)):
-      print("ERR647: Error, g2_brn {:0.2f} is bigger than g2_big_r {:0.2f}".format(g2_brn, g2_big_r))
+      six.print_(("ERR647: Error, g2_brn {:0.2f} is bigger than g2_big_r {:0.2f}".format(g2_brn, g2_big_r)))
       sys.exit(2)
   g1_param['positive_base_radius'] = g1_brp
   g1_param['negative_base_radius'] = g1_brn
@@ -652,12 +653,12 @@ def gear_profile_constraint_check(c):
   # hollow
   g1_h_delta = g1_thh*float(c['gear_hollow_height_pourcentage'])/100
   if(g1_h_delta<1.05*g1_rbr):
-    print("WARN218: Warning, g1_h_delta {:0.2f} is smaller than the router_bit_radius {:0.2f}. gear_hollow_height_pourcentage {:0.2f} should be set to {:0.2f}".format(g1_h_delta, g1_rbr, c['gear_hollow_height_pourcentage'], 100.0*g1_rbr/g1_thh))
+    six.print_(("WARN218: Warning, g1_h_delta {:0.2f} is smaller than the router_bit_radius {:0.2f}. gear_hollow_height_pourcentage {:0.2f} should be set to {:0.2f}".format(g1_h_delta, g1_rbr, c['gear_hollow_height_pourcentage'], 100.0*g1_rbr/g1_thh)))
     g1_h_delta = 1.05*g1_rbr
   g2_h_delta = g2_thh*float(c['second_gear_hollow_height_pourcentage'])/100
   if(g2_exist):
     if(g2_h_delta<1.05*g2_rbr):
-      print("WARN219: Warning, g2_h_delta {:0.2f} is smaller than the second_router_bit_radius {:0.2f}. second_gear_hollow_height_pourcentage {:0.2f} should be set to {:0.2f}".format(g2_h_delta, g2_rbr, c['second_gear_hollow_height_pourcentage'], 100.0*g2_rbr/g2_thh))
+      six.print_(("WARN219: Warning, g2_h_delta {:0.2f} is smaller than the second_router_bit_radius {:0.2f}. second_gear_hollow_height_pourcentage {:0.2f} should be set to {:0.2f}".format(g2_h_delta, g2_rbr, c['second_gear_hollow_height_pourcentage'], 100.0*g2_rbr/g2_thh)))
       g2_h_delta = 1.05*g2_rbr
   g1_hr = g1_dr - g1_as*g1_h_delta
   g2_hr = g2_dr - g2_as*g2_h_delta
@@ -703,7 +704,7 @@ def gear_profile_constraint_check(c):
   g1_ple = 0
   if(c['portion_tooth_nb']>0): # cut a portion of the first gear
     if(((g1_type=='e')or(g1_type=='i'))and(c['portion_tooth_nb']>=g1_n)):
-      print("ERR553: Error, the portion {:d} of gearwheel is bigger than the maximal number of teeth {:d}!".format(c['portion_tooth_nb'], g1_n))
+      six.print_(("ERR553: Error, the portion {:d} of gearwheel is bigger than the maximal number of teeth {:d}!".format(c['portion_tooth_nb'], g1_n)))
       sys.exit(2)
     g1_ptn = c['portion_tooth_nb']
     g1_pfe = c['portion_first_end']
@@ -942,7 +943,7 @@ def gear_profile_simulation_A(c):
   g2_rotation_speed_curve_table = []
   tangential_friction_curve_table = []
   ### start Tkinter
-  tk_root = Tkinter.Tk()
+  tk_root = tkinter.Tk()
   my_canvas = cnc25d_api.Two_Canvas(tk_root)
   # callback functions for display_backend
   def sub_canvas_graphics(ai_rotation_direction, ai_angle_position):
diff --git a/cnc25d/gear_profile_outline.py b/cnc25d/gear_profile_outline.py
index 62333e2..bd33af4 100644
--- a/cnc25d/gear_profile_outline.py
+++ b/cnc25d/gear_profile_outline.py
@@ -29,7 +29,7 @@
 # header for Python / FreeCAD compatibility
 ################################################################
 
-import cnc25d_api
+from . import cnc25d_api
 #cnc25d_api.importing_freecad()
 
 #print("FreeCAD.Version:", FreeCAD.Version())
@@ -54,7 +54,8 @@
 #import svgwrite
 #from dxfwrite import DXFEngine
 # cnc25d
-import small_geometry # use some well-tested functions from the internal of the cnc25d_api
+from . import small_geometry # use some well-tested functions from the internal of the cnc25d_api
+import six
 
 ################################################################
 # module variable
@@ -91,8 +92,8 @@ def involute_to_circle(ai_center, ai_base_radius, ai_initial_angle, ai_orientati
   u = ai_parameter
   # check the parameter
   if(u<0):
-    print("ERR099: Error, the parameter of the involute_to_circle must be positive {:0.8f}".format(u))
-    print("dbg887: ai_center {:0.2f} {:0.2f}  ai_base_radius {:0.2f}  ai_initial_angle {:0.2f}  ai_orientation {:d}  ai_parameter {:0.2f}".format(ai_center[0], ai_center[1], ai_base_radius, ai_initial_angle, ai_orientation, ai_parameter))
+    six.print_(("ERR099: Error, the parameter of the involute_to_circle must be positive {:0.8f}".format(u)))
+    six.print_(("dbg887: ai_center {:0.2f} {:0.2f}  ai_base_radius {:0.2f}  ai_initial_angle {:0.2f}  ai_orientation {:d}  ai_parameter {:0.2f}".format(ai_center[0], ai_center[1], ai_base_radius, ai_initial_angle, ai_orientation, ai_parameter)))
     sys.exit(2)
   # involute_to_circle of center (0,0), radius 1 and initial_angle = 0 with the parameter u
   px0 = math.cos(u)+u*math.sin(u)
@@ -133,7 +134,7 @@ def search_point_of_involute_to_circle(ai_center, ai_base_radius, ai_initial_ang
   R = ai_altitude
   # check the paramter
   if(R<B):
-    print("ERR098: Error, the altitude {:0.2f} is smaller than the base_diameter {:0.2f}".format(R, B))
+    six.print_(("ERR098: Error, the altitude {:0.2f} is smaller than the base_diameter {:0.2f}".format(R, B)))
     sys.exit(2)
   ### method 1
   ## converge to P
@@ -234,17 +235,17 @@ def calc_low_level_gear_parameters(ai_param):
   # tooth height check
   #
   if(g_ks*(g_pr-g_dr)<0):
-    print("ERR985: Error, g_pr {:0.2f} and g_dr {:0.2f} are not in the correct order!".format(g_pr, g_dr))
+    six.print_(("ERR985: Error, g_pr {:0.2f} and g_dr {:0.2f} are not in the correct order!".format(g_pr, g_dr)))
     sys.exit(2)
   if(g_ks*(g_ar-g_pr)<0):
-    print("ERR986: Error, g_pr {:0.2f} and g_ar {:0.2f} are not in the correct order!".format(g_pr, g_ar))
+    six.print_(("ERR986: Error, g_pr {:0.2f} and g_ar {:0.2f} are not in the correct order!".format(g_pr, g_ar)))
     sys.exit(2)
   # involute resolution check
   if(g_irp<1):
-    print("ERR786: Error, g_irp {:d} must be equal or bigger than 1!".format(g_irp))
+    six.print_(("ERR786: Error, g_irp {:d} must be equal or bigger than 1!".format(g_irp)))
     sys.exit(2)
   if(g_irn<1):
-    print("ERR787: Error, g_irn {:d} must be equal or bigger than 1!".format(g_irn))
+    six.print_(("ERR787: Error, g_irn {:d} must be equal or bigger than 1!".format(g_irn)))
     sys.exit(2)
   ### calculation of the low level parameters
   r_low_parameters = ()
@@ -255,13 +256,13 @@ def calc_low_level_gear_parameters(ai_param):
     pi_module_angle = ai_param['pi_module_angle']
     # intersection of positive involute and the primitive circle
     if(g_brp>g_pr-radian_epsilon):
-      print("ERR987: Error, g_brp {:0.2f} is bigger than g_pr {:0.2f}!".format(g_brp, g_pr))
+      six.print_(("ERR987: Error, g_brp {:0.2f} is bigger than g_pr {:0.2f}!".format(g_brp, g_pr)))
       sys.exit(2)
     (ippu, ippa, ippx, ippy, ippti) = search_point_of_involute_to_circle((g_ox, g_oy), g_brp, 0, 1, g_pr, initial_step, radian_epsilon_2)
     #ai_param['low_positive_primitive_angle'] = ippa
     # intersection of negative involute and the primitive circle
     if(g_brn>g_pr-radian_epsilon):
-      print("ERR988: Error, g_brn {:0.2f} is bigger than g_pr {:0.2f}!".format(g_brp, g_pr))
+      six.print_(("ERR988: Error, g_brn {:0.2f} is bigger than g_pr {:0.2f}!".format(g_brp, g_pr)))
       sys.exit(2)
     (inpu, inpa, inpx, inpy, inpti) = search_point_of_involute_to_circle((g_ox, g_oy), g_brn, 0, -1, g_pr, initial_step, radian_epsilon_2)
     #ai_param['low_negative_primitive_angle'] = inpa
@@ -297,12 +298,12 @@ def calc_low_level_gear_parameters(ai_param):
     top_land = pi_module_angle*g_adp-(addendum_positive_involute+addendum_negative_involute)
     #print("dbg298: top_land", top_land) [bug] dxf-top_land is negative whereas this top_land is still positive [todo] fix it
     if(top_land*g_ar<radian_epsilon): # a bit stricter than 0
-      print("ERR989: Error, the top_land {:0.2f} is negative or too small!".format(top_land))
-      print("dbg553: g_pr {:0.3f}  g_brp {:0.3f}  g_brn {:0.3f}".format(g_pr, g_brp, g_brn))
+      six.print_(("ERR989: Error, the top_land {:0.2f} is negative or too small!".format(top_land)))
+      six.print_(("dbg553: g_pr {:0.3f}  g_brp {:0.3f}  g_brn {:0.3f}".format(g_pr, g_brp, g_brn)))
       sys.exit(2)
     bottom_land = pi_module_angle*(1-g_adp)-(dedendum_positive_involute+dedendum_negative_involute)
     if(bottom_land*g_dr<2.1*g_rbr): # a bit stricter than router_bit_radius
-      print("ERR990: Error, the bottom_land {:0.2f} is negative or too small compare to the router_bit_radius {:0.2f} ({:0.2f} < {:0.2f})!".format(bottom_land, g_rbr, bottom_land*g_dr, 2*g_rbr))
+      six.print_(("ERR990: Error, the bottom_land {:0.2f} is negative or too small compare to the router_bit_radius {:0.2f} ({:0.2f} < {:0.2f})!".format(bottom_land, g_rbr, bottom_land*g_dr, 2*g_rbr)))
       sys.exit(2)
     ai_param['top_land'] = top_land
     ai_param['bottom_land'] = bottom_land
@@ -370,17 +371,17 @@ def calc_low_level_gear_parameters(ai_param):
     #bottom_land_length = bottom_land * g_dr
     bottom_land_length = 2 * g_dr * math.sin(bottom_land/2)
     if((g_ks*i1_dtri>0)or(g_ks*i2_dtri<0)):
-      print("ERR234: Error, i1_dtri {:0.3f} >0 or i2_dtri {:0.3f} < 0".format(g_ks*i1_dtri, g_ks*i2_dtri))
+      six.print_(("ERR234: Error, i1_dtri {:0.3f} >0 or i2_dtri {:0.3f} < 0".format(g_ks*i1_dtri, g_ks*i2_dtri)))
       sys.exit(2)
     cos_i1_dtri = math.cos(i1_dtri)
     cos_i2_dtri = math.cos(i2_dtri)
     if((cos_i1_dtri<radian_epsilon)or(cos_i1_dtri<radian_epsilon)):
-      print("ERR632: Error, i1_dtri {:0.3f} or i2_dtri {:0.3f} are too closed to pi/2 or even bigger!".format(i1_dtri, i2_dtri))
+      six.print_(("ERR632: Error, i1_dtri {:0.3f} or i2_dtri {:0.3f} are too closed to pi/2 or even bigger!".format(i1_dtri, i2_dtri)))
       sys.exit(2)
     ABh = bottom_land_length-i1_thickness/cos_i1_dtri-i2_thickness/cos_i2_dtri # see documentation graphic of hollow optimization
     ## check bottom_land with g_stp and g_stn
     if(ABh<radian_epsilon):
-      print("ERR381: Error, the bottom {:0.3f} is too small because of g_stp {:0.3f} and g_stn {:0.3f}".format(ABh, g_stp, g_stn))
+      six.print_(("ERR381: Error, the bottom {:0.3f} is too small because of g_stp {:0.3f} and g_stn {:0.3f}".format(ABh, g_stp, g_stn)))
       sys.exit(2)
     a = math.pi/2-abs(i1_dtri) - g_ks * bottom_land/2
     b = math.pi/2-abs(i2_dtri) - g_ks * bottom_land/2
@@ -402,11 +403,11 @@ def calc_low_level_gear_parameters(ai_param):
       AI2 = ABh*math.sin(b)/math.sin(AIB)
       BI2 = ABh*math.sin(a)/math.sin(AIB)
       if(abs(AI2-AI)>radian_epsilon):
-        print("ERR972: Error, AI {:0.3f} and AI2 {:0.3f} are not equal".format(AI, AI2))
-        print("dbg647: a {:0.3f}  b {:0.3f}  AIB {:0.3f}  ABh {:0.3f}".format(a, b, AIB, ABh))
+        six.print_(("ERR972: Error, AI {:0.3f} and AI2 {:0.3f} are not equal".format(AI, AI2)))
+        six.print_(("dbg647: a {:0.3f}  b {:0.3f}  AIB {:0.3f}  ABh {:0.3f}".format(a, b, AIB, ABh)))
         sys.exit(2)
       if(abs(BI2-BI)>radian_epsilon):
-        print("ERR973: Error, BI {:0.3f} and BI2 {:0.3f} are not equal".format(BI, BI2))
+        six.print_(("ERR973: Error, BI {:0.3f} and BI2 {:0.3f} are not equal".format(BI, BI2)))
         sys.exit(2)
       # select the method for AI and BI
       AI = AI2
@@ -459,11 +460,11 @@ def calc_low_level_gear_parameters(ai_param):
     top_land = pi_module*g_adp-(positive_addendum+negative_addendum)
     bottom_land = pi_module*(1-g_adp)-(positive_dedendum+negative_dedendum)
     if(top_land<radian_epsilon):
-      print("ERR858: Error, the linear gear top-land {:0.3f} is negative or too small!".format(top_land))
-      print("dbg455: g_sp {:0.3f}  g_sn {:0.3f}".format(g_sp, g_sn))
+      six.print_(("ERR858: Error, the linear gear top-land {:0.3f} is negative or too small!".format(top_land)))
+      six.print_(("dbg455: g_sp {:0.3f}  g_sn {:0.3f}".format(g_sp, g_sn)))
       sys.exit(2)
     if(bottom_land<2*g_rbr+radian_epsilon):
-      print("ERR859: Error, the linear gear bottom-land {:0.3f} is too small compare to the gear_router_bit_radius {:0.3f}".format(bottom_land, g_rbr))
+      six.print_(("ERR859: Error, the linear gear bottom-land {:0.3f} is too small compare to the gear_router_bit_radius {:0.3f}".format(bottom_land, g_rbr)))
       sys.exit(2)
     ai_param['top_land'] = top_land
     ai_param['bottom_land'] = bottom_land
@@ -497,7 +498,7 @@ def calc_low_level_gear_parameters(ai_param):
     cos_p = math.cos(g_sp)
     cos_n = math.cos(g_sn)
     if((cos_p<radian_epsilon)or(cos_n<radian_epsilon)):
-      print("ERR334: Error, g_sp {:0.3f}  or g_sn {:0.3f} are too closed to pi/2 or even bigger!".format(cos_p, cos_n))
+      six.print_(("ERR334: Error, g_sp {:0.3f}  or g_sn {:0.3f} are too closed to pi/2 or even bigger!".format(cos_p, cos_n)))
       sys.exit(2)
     ABh = bottom_land - g_stp/cos_p - g_stn/cos_n
     a = math.pi/2-abs(g_sp)
@@ -527,7 +528,7 @@ def calc_low_level_gear_parameters(ai_param):
     info_txt += "top land:          \t{:0.3f} (mm)  \t{:0.2f} %\n".format(top_land, 100*top_land/pi_module)
     info_txt += "bottom land:       \t{:0.3f} (mm)  \t{:0.2f} %\n".format(bottom_land, 100*bottom_land/pi_module)
   else:
-    print("ERR740: Error, the gear_type {:s} doesn't exist!".format(g_type))
+    six.print_(("ERR740: Error, the gear_type {:s} doesn't exist!".format(g_type)))
     sys.exit(2)
   #return
   r_cllgp = (make_low_parameters, info_txt)
@@ -571,7 +572,7 @@ def involute_outline(ai_ox, ai_oy, ai_base_radius, ai_offset, ai_sign, ai_u_nb,
     r_hollow_slope_A = ((p1x, p1y, 0), (p2x, p2y, ai_rbr))
     r_ti = p1t
   else:
-    print("ERR563: Error, the hollow end {:d} can only be 1 or -1".format(ai_he))
+    six.print_(("ERR563: Error, the hollow end {:d} can only be 1 or -1".format(ai_he)))
     sys.exit(2)
   #if(ai_dbg==0):
   #  print("dbg569: ai_dsl {:0.3f}  ai_hsl {:0.3f}  + {:0.3f}".format(ai_dsl, ai_hsl, ai_dsl + ai_hsl))
@@ -605,7 +606,7 @@ def half_hollow_outline(ai_tooth_angle, ai_sx, ai_sy, ai_si, ai_hrbr, ai_ham, ai
     tx = ai_ox + otl * math.cos(tox)
     ty = ai_oy + otl * math.sin(tox)
   else: # this case is very rare (maybe impossible because of the restriction on hrbr). So this code has not been tested yet!
-    print("dbg732: narrow half-hollow {:d}".format(fnl))
+    six.print_(("dbg732: narrow half-hollow {:d}".format(fnl)))
     ## P (identical with T)
     # angle poc
     pox = ai_tooth_angle - fnl * ai_ham
@@ -657,7 +658,7 @@ def gearwheel_profile_outline(ai_low_parameters, ai_angle_position):
   elif(gear_type=='i'):
     hgt = -1
   else:
-    print("ERR556: gear_type {:s} must be 'e' or 'i'".format(gear_type))
+    six.print_(("ERR556: gear_type {:s} must be 'e' or 'i'".format(gear_type)))
     sys.exit(2)
   # construct the final_outline
   r_final_outline = []
@@ -699,7 +700,7 @@ def gearwheel_profile_outline(ai_low_parameters, ai_angle_position):
     # check the optimization
     if(ho):
       if((abs(first_hollow_slope_A[1][0]-second_hollow_slope_A[0][0])>radian_epsilon)or(abs(first_hollow_slope_A[1][1]-second_hollow_slope_A[0][1])>radian_epsilon)):
-        print("ERR582: the hollow optimization intersection is wrong: x1 {:0.3f}   x2 {:0.3f}  y1 {:0.3f}   y2 {:0.3f}".format(first_hollow_slope_A[1][0], second_hollow_slope_A[0][0], first_hollow_slope_A[1][1], second_hollow_slope_A[0][1]))
+        six.print_(("ERR582: the hollow optimization intersection is wrong: x1 {:0.3f}   x2 {:0.3f}  y1 {:0.3f}   y2 {:0.3f}".format(first_hollow_slope_A[1][0], second_hollow_slope_A[0][0], first_hollow_slope_A[1][1], second_hollow_slope_A[0][1])))
         #hollow_A = []
         #hollow_A.extend(first_hollow_slope_A)
         #hollow_A.extend(second_hollow_slope_A)
@@ -849,7 +850,7 @@ def gearbar_profile_outline(ai_low_parameters, ai_tangential_position):
     l_negative_slope = slope_outline(g_ox, g_oy, g_bi, gb_n_offset, g_sn, -1, g_aln, g_dln, g_hln, g_stn, g_rbr, tangential_position+pi_module) # negative slope
     if(ho):
       if((abs(l_negative_slope[0][0]-l_positive_slope[1][0])>radian_epsilon)or(abs(l_negative_slope[0][1]-l_positive_slope[1][1])>radian_epsilon)):
-        print("ERR583: the gearbar hollow optimization intersection is wrong: x1 {:0.3f}   x2 {:0.3f}  y1 {:0.3f}   y2 {:0.3f}".format(l_positive_slope[1][0], l_negative_slope[0][0], l_positive_slope[1][1], l_negative_slope[0][1]))
+        six.print_(("ERR583: the gearbar hollow optimization intersection is wrong: x1 {:0.3f}   x2 {:0.3f}  y1 {:0.3f}   y2 {:0.3f}".format(l_positive_slope[1][0], l_negative_slope[0][0], l_positive_slope[1][1], l_negative_slope[0][1])))
         sys.exit(2)
     gearbar_A.extend(l_positive_slope)
     if(ho):
@@ -935,12 +936,12 @@ def calc_real_force_angle(ai_g1_type, ai_g1_pr, ai_g1_br, ai_g2_type, ai_g2_pr,
     r_real_force_angle = math.acos(float(ai_g1_br+ai_g2_br)/(ai_g1_pr+ai_g2_pr+ai_aal))
   elif((ai_g1_type=='i')and(ai_g2_type=='e')): # i-e
     if(ai_g2_pr>ai_g1_pr):
-      print("ERR546: Error, the gearring radius {:0.3f} must be bigger gearwheel radius {:0.3f}".format(ai_g1_pr, ai_g2_pr))
+      six.print_(("ERR546: Error, the gearring radius {:0.3f} must be bigger gearwheel radius {:0.3f}".format(ai_g1_pr, ai_g2_pr)))
       sys.exit(2)
     r_real_force_angle = math.acos(float(ai_g1_br-ai_g2_br)/(ai_g1_pr-(ai_g2_pr+ai_aal)))
   elif((ai_g1_type=='e')and(ai_g2_type=='i')): # e-i
     if(ai_g1_pr>ai_g2_pr):
-      print("ERR547: Error, the gearring radius {:0.3f} must be bigger gearwheel radius {:0.3f}".format(ai_g2_pr, ai_g1_pr))
+      six.print_(("ERR547: Error, the gearring radius {:0.3f} must be bigger gearwheel radius {:0.3f}".format(ai_g2_pr, ai_g1_pr)))
       sys.exit(2)
     r_real_force_angle = math.acos(float(ai_g2_br-ai_g1_br)/(ai_g2_pr-(ai_g1_pr+ai_aal)))
   elif((ai_g1_type=='l')and(ai_g2_type=='e')): # l-e
@@ -948,7 +949,7 @@ def calc_real_force_angle(ai_g1_type, ai_g1_pr, ai_g1_br, ai_g2_type, ai_g2_pr,
   elif((ai_g1_type=='e')and(ai_g2_type=='l')): # e-l
     r_real_force_angle = ai_g2_sa
   else:
-    print("ERR221: Error, the gear type combination {:s}-{:s} does not exist!".format(ai_g1_type, ai_g2_type))
+    six.print_(("ERR221: Error, the gear type combination {:s}-{:s} does not exist!".format(ai_g1_type, ai_g2_type)))
     sys.exit(2)
   return(r_real_force_angle)
 
@@ -1047,7 +1048,7 @@ def pre_g2_position_calculation(ai_g1_param, ai_g2_param, ai_aal, ai_g1g2_a, ai_
   # alternative for AB
   AB2 = math.sqrt((g2_ox-g1_ox)**2+(g2_oy-g1_oy)**2)
   if(abs(AB2-AB)>radian_epsilon):
-    print("ERR414: Error with the calculation of AB {:0.3f} or {:0.3f}".format(AB, AB2))
+    six.print_(("ERR414: Error with the calculation of AB {:0.3f} or {:0.3f}".format(AB, AB2)))
     sys.exit(2)
   # KL (see documentation  graphic gear_position.svg), used only with e-e, e-i and i-e
   KL_rp = math.sin(rfa_rp)*AB
@@ -1056,10 +1057,10 @@ def pre_g2_position_calculation(ai_g1_param, ai_g2_param, ai_aal, ai_g1g2_a, ai_
     KL_rp2 = math.sqrt(AB**2 - (g1_ks*g1_br_rp+g2_ks*g2_br_rp)**2) # kind of pythagor
     KL_rn2 = math.sqrt(AB**2 - (g1_ks*g1_br_rn+g2_ks*g2_br_rn)**2) # kind of pythagor
     if(abs(KL_rp2-KL_rp)>radian_epsilon):
-      print("ERR815: Error, KL_rp {:0.3f} and KL_rp2 {:0.3f} are too different!".format(KL_rp, KL_rp2))
+      six.print_(("ERR815: Error, KL_rp {:0.3f} and KL_rp2 {:0.3f} are too different!".format(KL_rp, KL_rp2)))
       sys.exit(2)
     if(abs(KL_rn2-KL_rn)>radian_epsilon):
-      print("ERR876: Error, KL_rn {:0.3f} and KL_rn2 {:0.3f} are too different!".format(KL_rn, KL_rn2))
+      six.print_(("ERR876: Error, KL_rn {:0.3f} and KL_rn2 {:0.3f} are too different!".format(KL_rn, KL_rn2)))
       sys.exit(2)
   # KE, BE, BD
   KE1_rp = 0 # default value for g_type = 'e' or 'i'
@@ -1219,7 +1220,7 @@ def g2_position_calculation(ai_place_low_param, ai_rotation_direction, ai_g1_pos
     ABC = 0
   elif(min_ABC+med_ABC<max_ABC):
     print("ERR478: Error of length in the triangle ABC")
-    print("dbg637: AB {:0.3f}  AC {:0.3f}  BC {:0.3f}".format(AB, AC, BC))
+    six.print_(("dbg637: AB {:0.3f}  AC {:0.3f}  BC {:0.3f}".format(AB, AC, BC)))
     sys.exit(20)
   else:
     # law of cosine (Al-Kashi) in ABC
@@ -1241,11 +1242,11 @@ def g2_position_calculation(ai_place_low_param, ai_rotation_direction, ai_g1_pos
     ABC = math.copysign(ABC, ABC2)
     # check BAC and BAC2
     if(abs(BAC2-BAC)>radian_epsilon_1000):
-      print("ERR689: Error in the calculation of BAC {:0.3f} or BAC2 {:0.3f} !".format(BAC, BAC2))
+      six.print_(("ERR689: Error in the calculation of BAC {:0.3f} or BAC2 {:0.3f} !".format(BAC, BAC2)))
       sys.exit(2)
     # check ABC and ABC2
     if(abs(ABC2-ABC)>radian_epsilon_1000):
-      print("ERR688: Error in the calculation of ABC {:0.3f} or ABC2 {:0.3f} !".format(ABC, ABC2))
+      six.print_(("ERR688: Error in the calculation of ABC {:0.3f} or ABC2 {:0.3f} !".format(ABC, ABC2)))
       sys.exit(2)
   #print("dbg334: BAC: {:0.3f}".format(BAC))
   #print("dbg335: ABC: {:0.3f}".format(ABC))
@@ -1255,7 +1256,7 @@ def g2_position_calculation(ai_place_low_param, ai_rotation_direction, ai_g1_pos
   if((g1_type=='e')or(g1_type=='i')):
     g1_sra2 = rd*g1_ks*math.atan(g1_contact_u)
     if(abs(g1_sra2-g1_sra)>radian_epsilon_100):
-      print("ERR417: Error in calculation of g1_sra {:0.3f} or g1_sra2 {:0.3f}".format(g1_sra, g1_sra2))
+      six.print_(("ERR417: Error in calculation of g1_sra {:0.3f} or g1_sra2 {:0.3f}".format(g1_sra, g1_sra2)))
       sys.exit(2)
   ## speed of c1 (contact point of g1)
   # c1 speed
@@ -1279,10 +1280,10 @@ def g2_position_calculation(ai_place_low_param, ai_rotation_direction, ai_g1_pos
     if((g1_type=='e')or(g1_type=='i')):
       g2_contact_u1 = g1_ks*float(KL - g2_ks*g1_contact_u*g1_br)/g2_br
       if(abs(g2_contact_u2-g2_contact_u1)>radian_epsilon_10):
-        print("ERR331: Error in the calculation of g2_contact_u1 {:0.3f} or g2_contact_u2 {:0.3f}".format(g2_contact_u1, g2_contact_u2))
+        six.print_(("ERR331: Error in the calculation of g2_contact_u1 {:0.3f} or g2_contact_u2 {:0.3f}".format(g2_contact_u1, g2_contact_u2)))
         sys.exit(2)
       if(abs(g2_contact_u3-g2_contact_u1)>radian_epsilon_10):
-        print("ERR332: Error in the calculation of g2_contact_u1 {:0.3f} or g2_contact_u3 {:0.3f}".format(g2_contact_u1, g2_contact_u3))
+        six.print_(("ERR332: Error in the calculation of g2_contact_u1 {:0.3f} or g2_contact_u3 {:0.3f}".format(g2_contact_u1, g2_contact_u3)))
         sys.exit(2)
     g2_contact_u = g2_contact_u3 # select the method for g2_contact_u
     ## c2_position
@@ -1291,7 +1292,7 @@ def g2_position_calculation(ai_place_low_param, ai_rotation_direction, ai_g1_pos
     (c2x, c2y, t2i) = sample_of_gear_tooth_profile((g2_ox, g2_oy), g2_br, g2_position+g2_lo, -1*rd*g1_ks, 0, g2_contact_u)
     #print("dbg326: g2_position {:0.3f}  g2_lo {:0.3f}  g2_contact_u {:0.3f}  t2i {:0.3f}".format(g2_position, g2_lo, g2_contact_u, t2i))
     if(abs(math.fmod(ti-t2i+4.5*math.pi, math.pi) - 0.5*math.pi)>radian_epsilon_1000):
-      print("ERR874: Error, the tangents ti {:0.3f} and t2i {:0.3f} are not equal (modulo pi)".format(ti, t2i))
+      six.print_(("ERR874: Error, the tangents ti {:0.3f} and t2i {:0.3f} are not equal (modulo pi)".format(ti, t2i)))
       sys.exit(2)
     #print("dbg632: g2_ox {:0.3f}  g2_oy {:0.3f}  g2_br {:0.3f}".format(g2_ox, g2_oy, g2_br))
     ## speed of c2 (contact point of g2)
@@ -1301,8 +1302,8 @@ def g2_position_calculation(ai_place_low_param, ai_rotation_direction, ai_g1_pos
     # alternative
     c2_speed_tangential2 = rd*g1_ks*c2_speed_radial*g2_contact_u
     if(abs(c2_speed_tangential2-c2_speed_tangential)>radian_epsilon_10):
-      print("ERR336: Error in the calculation of c2_speed_tangential {:0.3f} or c2_speed_tangential2 {:0.3f}".format(c2_speed_tangential, c2_speed_tangential2))
-      print("dbg967: radian_epsilon_1000 {:0.6f}  radian_epsilon_100000 {:0.6f}".format(radian_epsilon_1000, radian_epsilon_100000))
+      six.print_(("ERR336: Error in the calculation of c2_speed_tangential {:0.3f} or c2_speed_tangential2 {:0.3f}".format(c2_speed_tangential, c2_speed_tangential2)))
+      six.print_(("dbg967: radian_epsilon_1000 {:0.6f}  radian_epsilon_100000 {:0.6f}".format(radian_epsilon_1000, radian_epsilon_100000)))
       sys.exit(2)
     g2_rotation_speed = float(c2_speed)/BC
   elif(g2_type=='l'): # linear-gear (aka gearbar)
@@ -1314,7 +1315,7 @@ def g2_position_calculation(ai_place_low_param, ai_rotation_direction, ai_g1_pos
     c2y = g2_oy + rd*BE2*math.sin(g2_bi-math.pi/2) + dc*math.sin(g2_bi-math.pi/2-rd*g2_sa)
     #print("dbg989: g2_position {:0.3f}   dc {:0.3f}".format(g2_position, dc))
     if(abs(math.fmod(ti - (g2_bi-1*rd*g2_sa)+4.5*math.pi, math.pi)-0.5*math.pi)>radian_epsilon_1000):
-      print("ERR875: Error, the tangents ti {:0.3f} and slope g2_sa {:0.3f} are not equal (modulo pi)".format(ti, g2_sa))
+      six.print_(("ERR875: Error, the tangents ti {:0.3f} and slope g2_sa {:0.3f} are not equal (modulo pi)".format(ti, g2_sa)))
       sys.exit(2)
     c2_speed_radial = c1_speed_radial
     c2_speed = float(c2_speed_radial)/math.cos(g2_sa)
@@ -1366,7 +1367,7 @@ def info_on_real_force_angle(ai_g1_param, ai_g2_param, ai_sys_param, ai_rotation
   elif(ai_rotation_direction==-1):
     rotation_name = 'Negative'
   else:
-    print("ERR663: Error, ai_rotation_direction {:d} can only be 1 or -1!".format(ai_rotation_direction))
+    six.print_(("ERR663: Error, ai_rotation_direction {:d} can only be 1 or -1!".format(ai_rotation_direction)))
     sys.exit(2)
   # get the interesting high-level parameters
   rd = ai_rotation_direction
diff --git a/cnc25d/gearbar.py b/cnc25d/gearbar.py
index f21ba29..57e5f20 100644
--- a/cnc25d/gearbar.py
+++ b/cnc25d/gearbar.py
@@ -30,7 +30,8 @@
 # header for Python / FreeCAD compatibility
 ################################################################
 
-import cnc25d_api
+from . import cnc25d_api
+import six
 cnc25d_api.importing_freecad()
 
 #print("FreeCAD.Version:", FreeCAD.Version())
@@ -53,7 +54,7 @@
 #import svgwrite
 #from dxfwrite import DXFEngine
 # cnc25d
-import gear_profile
+from . import gear_profile
 
 ################################################################
 # inheritance from gear_profile
@@ -126,7 +127,7 @@ def gearbar_constraint_check(c):
   c['gearbar_hole_radius'] = float(c['gearbar_hole_diameter'])/2
   # c['gearbar_hole_height_position']
   if((c['gearbar_hole_height_position']+c['gearbar_hole_radius'])>c['gearbar_height']):
-    print("ERR215: Error, gearbar_hole_height_position {:0.3} and gearbar_hole_radius {:0.3f} are too big compare to gearbar_height {:0.3f} !".format(c['gearbar_hole_height_position'], c['gearbar_hole_radius'], c['gearbar_height']))
+    six.print_(("ERR215: Error, gearbar_hole_height_position {:0.3} and gearbar_hole_radius {:0.3f} are too big compare to gearbar_height {:0.3f} !".format(c['gearbar_hole_height_position'], c['gearbar_hole_radius'], c['gearbar_height'])))
     sys.exit(2)
   # c['gearbar_hole_increment']
   if(c['gearbar_hole_increment']==0):
@@ -156,7 +157,7 @@ def gearbar_constraint_check(c):
     bottom_land = gear_profile_parameters['g1_param']['bottom_land']
     top_land = gear_profile_parameters['g1_param']['top_land']
     if((top_land + full_positive_slope + bottom_land + full_negative_slope)!=c['pi_module']):
-      print("ERR269: Error with top_land {:0.3f}  full_positive_slope {:0.3f}  bottom_land {:0.3f}  full_negative_slope {:0.3f} and pi_module  {:0.3f}".format(top_land, full_positive_slope, bottom_land, full_negative_slope, c['pi_module']))
+      six.print_(("ERR269: Error with top_land {:0.3f}  full_positive_slope {:0.3f}  bottom_land {:0.3f}  full_negative_slope {:0.3f} and pi_module  {:0.3f}".format(top_land, full_positive_slope, bottom_land, full_negative_slope, c['pi_module'])))
       sys.exit(2)
     if(pfe==0):
       c['first_tooth_position'] = full_positive_slope + bottom_land + full_negative_slope + float(top_land)/2
@@ -168,7 +169,7 @@ def gearbar_constraint_check(c):
       c['first_tooth_position'] = float(bottom_land)/2 + full_negative_slope + float(top_land)/2
   else: # no gear_profile, just a circle
     if(c['gear_primitive_diameter']<radian_epsilon):
-      print("ERR885: Error, the no-gear-profile line outline length gear_primitive_diameter {:0.2f} is too small!".format(c['gear_primitive_diameter']))
+      six.print_(("ERR885: Error, the no-gear-profile line outline length gear_primitive_diameter {:0.2f} is too small!".format(c['gear_primitive_diameter'])))
       sys.exit(2)
     #c['g1_ix'] = c['center_position_x
     #c['g1_iy'] = c['center_position_y
@@ -180,11 +181,11 @@ def gearbar_constraint_check(c):
   ### check parameter coherence (part 2)
   # minimal_gear_profile_height
   if(c['minimal_gear_profile_height']<radian_epsilon):
-    print("ERR265: Error, minimal_gear_profile_height {:0.3f} is too small".format(c['minimal_gear_profile_height']))
+    six.print_(("ERR265: Error, minimal_gear_profile_height {:0.3f} is too small".format(c['minimal_gear_profile_height'])))
     sys.exit(2)
   # gearbar_hole_diameter
   if((c['gearbar_hole_height_position']+c['gearbar_hole_radius'])>c['minimal_gear_profile_height']):
-    print("ERR269: Error, gearbar_hole_height_position {:0.3f} and gearbar_hole_radius {:0.3f} are too big compare to minimal_gear_profile_height {:0.3f}".format(c['gearbar_hole_height_position'], c['gearbar_hole_radius'], c['minimal_gear_profile_height']))
+    six.print_(("ERR269: Error, gearbar_hole_height_position {:0.3f} and gearbar_hole_radius {:0.3f} are too big compare to minimal_gear_profile_height {:0.3f}".format(c['gearbar_hole_height_position'], c['gearbar_hole_radius'], c['minimal_gear_profile_height'])))
     sys.exit(2)
   # pi_module
   if(c['gearbar_hole_radius']>0):
diff --git a/cnc25d/gearring.py b/cnc25d/gearring.py
index 07c4bf6..bd952d0 100644
--- a/cnc25d/gearring.py
+++ b/cnc25d/gearring.py
@@ -30,7 +30,7 @@
 # header for Python / FreeCAD compatibility
 ################################################################
 
-import cnc25d_api
+from . import cnc25d_api
 #cnc25d_api.importing_freecad()
 
 #print("FreeCAD.Version:", FreeCAD.Version())
@@ -53,8 +53,9 @@
 #import svgwrite
 #from dxfwrite import DXFEngine
 # cnc25d
-import gear_profile
-import gearwheel # gearwheel.marked_circle_crenel()
+from . import gear_profile
+from . import gearwheel # gearwheel.marked_circle_crenel()
+import six
 
 
 ################################################################
@@ -176,7 +177,7 @@ def gearring_constraint_check(c):
     gear_module = gear_profile_parameters['g1_param']['module']
   else: # no gear_profile, just a circle
     if(c['gear_primitive_diameter']<radian_epsilon):
-      print("ERR176: Error, the no-gear-profile circle outline diameter gear_primitive_diameter {:0.2f} is too small!".format(c['gear_primitive_diameter']))
+      six.print_(("ERR176: Error, the no-gear-profile circle outline diameter gear_primitive_diameter {:0.2f} is too small!".format(c['gear_primitive_diameter'])))
       sys.exit(2)
     c['g1_ix'] = c['center_position_x']
     c['g1_iy'] = c['center_position_y']
@@ -195,7 +196,7 @@ def gearring_constraint_check(c):
   c['holder_crenel_half_width'] = float(c['holder_crenel_width'])/2
   holder_crenel_with_wall_half_width = c['holder_crenel_half_width'] + c['holder_crenel_skin_width']
   if(c['holder_radius']<holder_crenel_with_wall_half_width):
-    print("ERR213: Error, holder_radius {:0.3f} must be bigger than holder_crenel_with_wall_half_width {:0.3f}".format(c['holder_radius'], holder_crenel_with_wall_half_width))
+    six.print_(("ERR213: Error, holder_radius {:0.3f} must be bigger than holder_crenel_with_wall_half_width {:0.3f}".format(c['holder_radius'], holder_crenel_with_wall_half_width)))
     sys.exit(2)
   c['holder_crenel_half_angle'] = math.asin(float(holder_crenel_with_wall_half_width)/c['holder_radius'])
   c['holder_crenel_x_position'] = math.sqrt((c['holder_radius'])**2 - (holder_crenel_with_wall_half_width)**2)
@@ -211,7 +212,7 @@ def gearring_constraint_check(c):
   # c['holder_crenel_height
   if(c['holder_crenel_number']>0):
     if((0.9*holder_side_straigth_length)<c['holder_sr']):
-      print("ERR218: Error, the holder-crenel-wall-side height is too small: holder_side_straigth_length {:0.3f}  holder_smoothing_radius {:0.3f}".format(holder_side_straigth_length, c['holder_sr']))
+      six.print_(("ERR218: Error, the holder-crenel-wall-side height is too small: holder_side_straigth_length {:0.3f}  holder_smoothing_radius {:0.3f}".format(holder_side_straigth_length, c['holder_sr'])))
       #print("dbg214: holder_radius, holder_crenel_x_position:", c['holder_radius'], c['holder_crenel_x_position'])
       #print("dbg215: holder_maximal_height, additional_holder_maximal_height:", holder_maximal_height, additional_holder_maximal_height)
       #print("dbg216: holder_crenel_skin_width, holder_maximal_height_plus:", c['holder_crenel_skin_width'], c['holder_maximal_height_plus'])
@@ -221,44 +222,44 @@ def gearring_constraint_check(c):
       sys.exit(2)
   # c['holder_crenel_position']
   if(c['holder_crenel_position']<c['holder_hole_radius']):
-    print("ERR211: Error, holder_crenel_position {:0.3f} is too small compare to holder_hole_radius {:0.3f}".format(c['holder_crenel_position'], c['holder_hole_radius']))
+    six.print_(("ERR211: Error, holder_crenel_position {:0.3f} is too small compare to holder_hole_radius {:0.3f}".format(c['holder_crenel_position'], c['holder_hole_radius'])))
     sys.exit(2)
   # c['holder_crenel_width']
   if(c['holder_crenel_width']<2.1*c['holder_crenel_rbr']):
-    print("ERR215: Error, holder_crenel_width {:0.3} is too small compare to holder_crenel_router_bit_radius {:0.3f}".format(c['holder_crenel_width'], c['holder_crenel_rbr']))
+    six.print_(("ERR215: Error, holder_crenel_width {:0.3} is too small compare to holder_crenel_router_bit_radius {:0.3f}".format(c['holder_crenel_width'], c['holder_crenel_rbr'])))
     sys.exit(2)
   # hollow_circle and holder-hole
   if(c['maximal_gear_profile_radius']>(c['holder_hole_position_radius']-c['holder_hole_radius'])):
-    print("ERR303: Error, holder-hole are too closed from the gear_hollow_circle: maximal_gear_profile_radius {:0.3f}  holder_hole_position_radius {:0.3f}  holder_hole_radius {:0.3f}".format(c['maximal_gear_profile_radius'], c['holder_hole_position_radius'], c['holder_hole_radius']))
+    six.print_(("ERR303: Error, holder-hole are too closed from the gear_hollow_circle: maximal_gear_profile_radius {:0.3f}  holder_hole_position_radius {:0.3f}  holder_hole_radius {:0.3f}".format(c['maximal_gear_profile_radius'], c['holder_hole_position_radius'], c['holder_hole_radius'])))
     sys.exit(2)
   # holder_hole_mark_nb
   if((c['holder_hole_mark_nb']<0)or(c['holder_hole_mark_nb']>c['holder_crenel_number'])):
-    print("ERR294: Error, holder_hole_mark_nb {:d} is out of its range 0..{:d}".format(c['holder_hole_mark_nb'], c['holder_crenel_number']))
+    six.print_(("ERR294: Error, holder_hole_mark_nb {:d} is out of its range 0..{:d}".format(c['holder_hole_mark_nb'], c['holder_crenel_number'])))
     sys.exit(2)
   # holder_double_hole
   c['holder_double_hole_radius'] = c['holder_double_hole_diameter']/2.0
   c['holder_double_hole_position_radius'] = c['holder_hole_position_radius'] + c['holder_double_hole_position']
   if(c['holder_double_hole_length']<0):
-    print("ERR304: Error, holder_double_hole_length {:0.3f} should be positive".format(c['holder_double_hole_length']))
+    six.print_(("ERR304: Error, holder_double_hole_length {:0.3f} should be positive".format(c['holder_double_hole_length'])))
     sys.exit(2)
   elif(c['holder_double_hole_length']>0):
     if(c['holder_double_hole_radius']==0):
-      print("ERR308: Error, holder_double_hole_length {:0.3f} is positive whereas holder_double_hole_radius is set to zero".format(c['holder_double_hole_length']))
+      six.print_(("ERR308: Error, holder_double_hole_length {:0.3f} is positive whereas holder_double_hole_radius is set to zero".format(c['holder_double_hole_length'])))
       sys.exit(2)
   if(c['holder_double_hole_position']!=0):
     if(c['holder_double_hole_radius']==0):
-      print("ERR319: Error, holder_double_hole_position {:0.3f} is set whereas holder_double_hole_radius is still set to zero".format(c['holder_double_hole_position']))
+      six.print_(("ERR319: Error, holder_double_hole_position {:0.3f} is set whereas holder_double_hole_radius is still set to zero".format(c['holder_double_hole_position'])))
       sys.exit(2)
   if(c['holder_double_hole_radius']<0):
-    print("ERR322: Error, holder_double_hole_radius {:0.3f} must be positive or null".format(c['holder_double_hole_radius']))
+    six.print_(("ERR322: Error, holder_double_hole_radius {:0.3f} must be positive or null".format(c['holder_double_hole_radius'])))
     sys.exit(2)
   elif(c['holder_double_hole_radius']>0):
     if(c['holder_double_hole_length']==0):
-      print("ERR326: Error, holder_double_hole_length must be positive when holder_double_hole_radius {:0.3f} is positive".format(c['holder_double_hole_radius']))
+      six.print_(("ERR326: Error, holder_double_hole_length must be positive when holder_double_hole_radius {:0.3f} is positive".format(c['holder_double_hole_radius'])))
       sys.exit(2)
   # holder_double_hole_mark_nb
   if((c['holder_double_hole_mark_nb']<0)or(c['holder_double_hole_mark_nb']>c['holder_crenel_number'])):
-    print("ERR333: Error, holder_double_hole_mark_nb {:d} is out of its range 0..{:d}".format(c['holder_double_hole_mark_nb'], c['holder_crenel_number']))
+    six.print_(("ERR333: Error, holder_double_hole_mark_nb {:d} is out of its range 0..{:d}".format(c['holder_double_hole_mark_nb'], c['holder_crenel_number'])))
     sys.exit(2)
   ## holder_hole_B
   holder_hole_B_radius = float(c['holder_hole_B_diameter'])/2
@@ -266,7 +267,7 @@ def gearring_constraint_check(c):
   c['holder_hole_B_crenel_list_bis'] = [ 0 for i in range(c['holder_crenel_number']) ]
   for i in range(len(c['holder_hole_B_crenel_list'])):
     if((int(c['holder_hole_B_crenel_list'][i])<0)or(int(c['holder_hole_B_crenel_list'][i])>=c['holder_crenel_number'])):
-      print("ERR286: Error, the holder_hole_B_crenel_list index {:s} is out of the range 0..{:d}".format(c['holder_hole_B_crenel_list'][i], c['holder_crenel_number']))
+      six.print_(("ERR286: Error, the holder_hole_B_crenel_list index {:s} is out of the range 0..{:d}".format(c['holder_hole_B_crenel_list'][i], c['holder_crenel_number'])))
       sys.exit(2)
     c['holder_hole_B_crenel_list_bis'][int(c['holder_hole_B_crenel_list'][i])] = 1
   #print("dbg358: holder_hole_B_crenel_list_bis:", c['holder_hole_B_crenel_list_bis'])
@@ -293,7 +294,7 @@ def gearring_constraint_check(c):
   #print("dbg378: holder_smoothing_radius_list:", c['holder_smoothing_radius_list'])
   #
   if(c['holder_crenel_number_cut']>c['holder_crenel_number']):
-    print("ERR288: Error, holder_crenel_number_cut {:d} must be smaller than holder_crenel_number {:d}".format(c['holder_crenel_number_cut'], c['holder_crenel_number']))
+    six.print_(("ERR288: Error, holder_crenel_number_cut {:d} must be smaller than holder_crenel_number {:d}".format(c['holder_crenel_number_cut'], c['holder_crenel_number'])))
     sys.exit(2)
   ###
   return(c)
@@ -351,7 +352,7 @@ def gearring_2d_construction(c):
   elif(c['holder_crenel_number']>0):
     angle_incr = 2*math.pi/c['holder_crenel_number']
     if((angle_incr-2*c['holder_crenel_half_angle'])<math.pi/10):
-      print("ERR369: Error, no enough space between the crenel: angle_incr {:0.3f}  holder_crenel_half_angle {:0.3f}".format(angle_incr, c['holder_crenel_half_angle']))
+      six.print_(("ERR369: Error, no enough space between the crenel: angle_incr {:0.3f}  holder_crenel_half_angle {:0.3f}".format(angle_incr, c['holder_crenel_half_angle'])))
       sys.exit(2)
     holder_A = []
     first_angle = c['holder_position_angle'] - c['holder_crenel_half_angle']
diff --git a/cnc25d/gearwheel.py b/cnc25d/gearwheel.py
index 99c63ba..030c8d7 100644
--- a/cnc25d/gearwheel.py
+++ b/cnc25d/gearwheel.py
@@ -30,7 +30,7 @@
 # header for Python / FreeCAD compatibility
 ################################################################
 
-import cnc25d_api
+from . import cnc25d_api
 #cnc25d_api.importing_freecad()
 
 #print("FreeCAD.Version:", FreeCAD.Version())
@@ -53,7 +53,8 @@
 #import svgwrite
 #from dxfwrite import DXFEngine
 # cnc25d
-import gear_profile
+from . import gear_profile
+import six
 
 ################################################################
 # inheritance from gear_profile
@@ -157,24 +158,24 @@ def gearwheel_constraint_check(c):
     c['axle_rbr'] = c['cnc_router_bit_radius']
   # c['axle_type']
   if(not c['axle_type'] in ('none', 'circle', 'rectangle')):
-    print("ERR932: Error, axle_type {:s} is not valid!".format(c['axle_type']))
+    six.print_(("ERR932: Error, axle_type {:s} is not valid!".format(c['axle_type'])))
     sys.exit(2)
   # c['axle_x_width']
   axle_diameter = 0
   if(c['axle_type'] in('circle','rectangle')):
     if(c['axle_x_width']<2*c['axle_rbr']+radian_epsilon):
-      print("ERR663: Error, axle_x_width {:0.2f} is too small compare to axle_router_bit_radius {:0.2f}!".format(c['axle_x_width'], c['axle_rbr']))
+      six.print_(("ERR663: Error, axle_x_width {:0.2f} is too small compare to axle_router_bit_radius {:0.2f}!".format(c['axle_x_width'], c['axle_rbr'])))
       sys.exit(2)
     axle_diameter = c['axle_x_width']
     # c['axle_y_width']
     if(c['axle_type']=='rectangle'):
       if(c['axle_y_width']<2*c['axle_rbr']+radian_epsilon):
-        print("ERR664: Error, axle_y_width {:0.2f} is too small compare to axle_router_bit_radius {:0.2f}!".format(c['axle_y_width'], c['axle_rbr']))
+        six.print_(("ERR664: Error, axle_y_width {:0.2f} is too small compare to axle_router_bit_radius {:0.2f}!".format(c['axle_y_width'], c['axle_rbr'])))
         sys.exit(2)
       axle_diameter = math.sqrt(c['axle_x_width']**2+c['axle_y_width']**2)
   # crenel_type
   if((c['crenel_type']!='rectangle')and(c['crenel_type']!='circle')):
-    print("ERR213: Error, crenel_type {:s} is unknown".format(c['crenel_type']))
+    six.print_(("ERR213: Error, crenel_type {:s} is unknown".format(c['crenel_type'])))
     sys.exit(2)
   # crenel_number
   crenel_increment_angle = 0.0
@@ -182,13 +183,13 @@ def gearwheel_constraint_check(c):
     crenel_increment_angle = 2*math.pi/c['crenel_number']
   if(c['crenel_tooth_align']>0):
     if(c['crenel_number']>0):
-      print("ERR255: Error, crenel_tooth_align {:d} and crenel_number {:d} are set together".format(c['crenel_tooth_align'], c['crenel_number']))
+      six.print_(("ERR255: Error, crenel_tooth_align {:d} and crenel_number {:d} are set together".format(c['crenel_tooth_align'], c['crenel_number'])))
       sys.exit(2)
     if(c['crenel_angle']!=0):
-      print("ERR258: Error, crenel_tooth_align {:d} and crenel_angle {:0.3f} are set together".format(c['crenel_tooth_align'], c['crenel_angle']))
+      six.print_(("ERR258: Error, crenel_tooth_align {:d} and crenel_angle {:0.3f} are set together".format(c['crenel_tooth_align'], c['crenel_angle'])))
       sys.exit(2)
     if(c['gear_tooth_nb']==0):
-      print("ERR261: Error, crenel_tooth_align {:d} set and gear_tooth_nb {:d} is set to zero".format(c['crenel_tooth_align'], c['gear_tooth_nb']))
+      six.print_(("ERR261: Error, crenel_tooth_align {:d} set and gear_tooth_nb {:d} is set to zero".format(c['crenel_tooth_align'], c['gear_tooth_nb'])))
       sys.exit(2)
     c['crenel_number'] = int(c['gear_tooth_nb']/c['crenel_tooth_align'])
     c['crenel_angle'] = c['gear_initial_angle']
@@ -196,14 +197,14 @@ def gearwheel_constraint_check(c):
   #print("dbg266: crenel_number {:d}  crenel_angle {:0.3f}  crenel_increment_angle {:0.3f}".format(c['crenel_number'], c['crenel_angle'], crenel_increment_angle))
   # crenel_mark_nb
   if((c['crenel_mark_nb']<0)or(c['crenel_mark_nb']>c['crenel_number'])):
-    print("ERR233: Error, crenel_mark_nb {:d} is out of the range 0..{:d}".format(c['crenel_mark_nb'], c['crenel_number']))
+    six.print_(("ERR233: Error, crenel_mark_nb {:d} is out of the range 0..{:d}".format(c['crenel_mark_nb'], c['crenel_number'])))
     sys.exit(2)
   # crenel_diameter
   crenel_diameter = c['crenel_diameter']
   if(crenel_diameter==0):
     crenel_diameter = axle_diameter
   if(crenel_diameter<axle_diameter):
-    print("ERR212: Error, crenel_diameter {:0.3f} is smaller than the axle_diameter {:0.3f}".format(crenel_diameter, axle_diameter))
+    six.print_(("ERR212: Error, crenel_diameter {:0.3f} is smaller than the axle_diameter {:0.3f}".format(crenel_diameter, axle_diameter)))
     sys.exit(2)
   c['crenel_radius'] = crenel_diameter/2.0
   # c['gear_tooth_nb']
@@ -220,7 +221,7 @@ def gearwheel_constraint_check(c):
     c['addendum_radius'] = gear_profile_parameters['g1_param']['addendum_radius'] # for slice_xyz
   else: # no gear_profile, just a circle
     if(c['gear_primitive_diameter']<radian_epsilon):
-      print("ERR885: Error, the no-gear-profile circle outline diameter gear_primitive_diameter {:0.2f} is too small!".format(c['gear_primitive_diameter']))
+      six.print_(("ERR885: Error, the no-gear-profile circle outline diameter gear_primitive_diameter {:0.2f} is too small!".format(c['gear_primitive_diameter'])))
       sys.exit(2)
     c['g1_ix'] = c['center_position_x']
     c['g1_iy'] = c['center_position_y']
@@ -232,7 +233,7 @@ def gearwheel_constraint_check(c):
   if(c['axle_radius'] == 0): # if axle = none
     c['axle_radius'] = 1.0
   if(c['axle_radius']>minimal_gear_profile_radius-radian_epsilon): # non sense case
-    print("ERR218: Error, axle_radius {:0.3f} is bigger than minimal_gear_profile_radius {:0.3f}".format(c['axle_radius'], minimal_gear_profile_radius))
+    six.print_(("ERR218: Error, axle_radius {:0.3f} is bigger than minimal_gear_profile_radius {:0.3f}".format(c['axle_radius'], minimal_gear_profile_radius)))
     sys.exit(2)
   c['wheel_hollow_external_radius'] = c['wheel_hollow_external_diameter']/2.0
   c['wheel_hollow_internal_radius'] = c['wheel_hollow_internal_diameter']/2.0
@@ -240,63 +241,63 @@ def gearwheel_constraint_check(c):
     if(c['wheel_hollow_external_radius']==0): # set the default value
       c['wheel_hollow_external_radius'] = minimal_gear_profile_radius - 2.5*g1_m
       if(c['wheel_hollow_external_radius']<c['axle_radius']+2.1*c['wheel_hollow_rbr']): # remove the default value
-        print("WARN224: Warning, the wheel_hollow_external_radius default value {:0.3f} can not be set!".format(c['wheel_hollow_external_radius']))
+        six.print_(("WARN224: Warning, the wheel_hollow_external_radius default value {:0.3f} can not be set!".format(c['wheel_hollow_external_radius'])))
         c['wheel_hollow_external_radius'] = 0
         c['wheel_hollow_leg_number'] = 0
     if(c['wheel_hollow_internal_radius']==0): # set the default value
       #c['wheel_hollow_internal_radius'] = 2.0*c['axle_radius']
       c['wheel_hollow_internal_radius'] = c['axle_radius'] + 1.0*c['wheel_hollow_leg_width']
       if(c['wheel_hollow_internal_radius']>c['wheel_hollow_external_radius']-2.1*c['wheel_hollow_rbr']): # remove the default value
-        print("WARN228: Warning, the wheel_hollow_internal_radius default value {:0.3f} can not be set!".format(c['wheel_hollow_internal_radius']))
+        six.print_(("WARN228: Warning, the wheel_hollow_internal_radius default value {:0.3f} can not be set!".format(c['wheel_hollow_internal_radius'])))
         c['wheel_hollow_internal_radius'] = 0
         c['wheel_hollow_leg_number'] = 0
     if(c['wheel_hollow_router_bit_radius']==0): # set the default value
       c['wheel_hollow_rbr'] = (c['wheel_hollow_external_radius']-c['wheel_hollow_internal_radius'])/5.0
       if(c['wheel_hollow_rbr']<c['cnc_router_bit_radius']):
-        print("WARN233: Warning, the wheel_hollow_router_bit_radius default value {:0.3f} can not be set!".format(c['wheel_hollow_rbr']))
+        six.print_(("WARN233: Warning, the wheel_hollow_router_bit_radius default value {:0.3f} can not be set!".format(c['wheel_hollow_rbr'])))
         c['wheel_hollow_rbr'] = c['cnc_router_bit_radius']
   ### check parameter coherence (part 2)
   if(c['wheel_hollow_leg_number']>0):
     # wheel_hollow_external_diameter
     if(c['wheel_hollow_external_radius'] > minimal_gear_profile_radius-radian_epsilon):
-      print("ERR733: Error, wheel_hollow_external_radius {:0.2f} is bigger than the gear_hollow_radius {:0.2f}!".format(c['wheel_hollow_external_radius'], minimal_gear_profile_radius))
+      six.print_(("ERR733: Error, wheel_hollow_external_radius {:0.2f} is bigger than the gear_hollow_radius {:0.2f}!".format(c['wheel_hollow_external_radius'], minimal_gear_profile_radius)))
       sys.exit(2)
     if(c['wheel_hollow_external_radius'] < c['wheel_hollow_internal_radius']+2*c['wheel_hollow_rbr']+radian_epsilon):
-      print("ERR734: Error, wheel_hollow_external_radius {:0.2f} is too small compare to wheel_hollow_internal_radius {:0.2f} and wheel_hollow_router_bit_radius {:0.2f}!".format(c['wheel_hollow_external_radius'], c['wheel_hollow_internal_radius'], c['wheel_hollow_rbr']))
+      six.print_(("ERR734: Error, wheel_hollow_external_radius {:0.2f} is too small compare to wheel_hollow_internal_radius {:0.2f} and wheel_hollow_router_bit_radius {:0.2f}!".format(c['wheel_hollow_external_radius'], c['wheel_hollow_internal_radius'], c['wheel_hollow_rbr'])))
       sys.exit(2)
     # wheel_hollow_leg_width
     if(c['wheel_hollow_leg_width']<radian_epsilon):
-      print("ERR735: Error, wheel_hollow_leg_width {:0.2f} is too small!".format(c['wheel_hollow_leg_width']))
+      six.print_(("ERR735: Error, wheel_hollow_leg_width {:0.2f} is too small!".format(c['wheel_hollow_leg_width'])))
       sys.exit(2)
     # wheel_hollow_internal_diameter
     if(c['wheel_hollow_internal_radius']<c['axle_radius']+radian_epsilon):
-      print("ERR736: Error, wheel_hollow_internal_radius {:0.2f} is too small compare to axle_radius {:0.2f}!".format(c['wheel_hollow_internal_radius'], c['axle_radius']))
+      six.print_(("ERR736: Error, wheel_hollow_internal_radius {:0.2f} is too small compare to axle_radius {:0.2f}!".format(c['wheel_hollow_internal_radius'], c['axle_radius'])))
       sys.exit(2)
     if(2*c['wheel_hollow_internal_radius']<c['wheel_hollow_leg_width']+2*radian_epsilon):
-      print("ERR736: Error, wheel_hollow_internal_radius {:0.2f} is too small compare to wheel_hollow_leg_width {:0.2f}!".format(c['wheel_hollow_internal_radius'], c['wheel_hollow_leg_width']))
+      six.print_(("ERR736: Error, wheel_hollow_internal_radius {:0.2f} is too small compare to wheel_hollow_leg_width {:0.2f}!".format(c['wheel_hollow_internal_radius'], c['wheel_hollow_leg_width'])))
       sys.exit(2)
   if(c['crenel_number']>0):
     #print("dbg305: crenel_type {:s}".format(c['crenel_type']))
     if(c['crenel_type']=='rectangle'):
       if(math.sqrt((c['crenel_radius']+c['crenel_height'])**2+(c['crenel_width']/2)**2)> minimal_gear_profile_radius-radian_epsilon):
-        print("ERR298: Error, crenel_radius {:0.3f}, crenel_height {:0.3f} or crenel_width {:0.3f} are too big compare to minimal_gear_profile_radius {:0.3f}".format(c['crenel_radius'], c['crenel_height'], c['crenel_width'], minimal_gear_profile_radius))
+        six.print_(("ERR298: Error, crenel_radius {:0.3f}, crenel_height {:0.3f} or crenel_width {:0.3f} are too big compare to minimal_gear_profile_radius {:0.3f}".format(c['crenel_radius'], c['crenel_height'], c['crenel_width'], minimal_gear_profile_radius)))
         sys.exit(2)
     elif(c['crenel_type']=='circle'):
       if((c['crenel_radius']+c['crenel_width']/2)>minimal_gear_profile_radius-radian_epsilon):
-        print("ERR311: Error, crenel_radius {:0.3f} or crenel_width {:0.3f} are too big compare to minimal_gear_profile_radius {:0.3f}".format(c['crenel_radius'], c['crenel_width'], minimal_gear_profile_radius))
+        six.print_(("ERR311: Error, crenel_radius {:0.3f} or crenel_width {:0.3f} are too big compare to minimal_gear_profile_radius {:0.3f}".format(c['crenel_radius'], c['crenel_width'], minimal_gear_profile_radius)))
         sys.exit(2)
     if(c['crenel_radius']<radian_epsilon):
-      print("ERR301: Error, the crenel_radius {:0.3f} is too small".format(c['crenel_radius']))
+      six.print_(("ERR301: Error, the crenel_radius {:0.3f} is too small".format(c['crenel_radius'])))
       sys.exit(2)
     if(abs(c['crenel_width'])>2*c['crenel_radius']-radian_epsilon):
-      print("ERR304: Error, crenel_width {:0.3f} is too big compare to crenel_radius {:0.3f}".format(c['crenel_width'], c['crenel_radius']))
+      six.print_(("ERR304: Error, crenel_width {:0.3f} is too big compare to crenel_radius {:0.3f}".format(c['crenel_width'], c['crenel_radius'])))
       sys.exit(2)
     c['crenel_half_width_angle'] = math.asin(c['crenel_width']/(2*c['crenel_radius']))
     if(c['crenel_half_width_angle']*2.2>crenel_increment_angle):
-      print("ERR305: Error, the crenel_increment_angle {:0.3f} or crenel_width {:0.3f} are too big!".format(crenel_increment_angle, c['crenel_width']))
+      six.print_(("ERR305: Error, the crenel_increment_angle {:0.3f} or crenel_width {:0.3f} are too big!".format(crenel_increment_angle, c['crenel_width'])))
       sys.exit(2)
     if(c['crenel_width']<3.2*c['crenel_rbr']):
-      print("ERR308: Error, crenel_width {:0.3f} is too small compare to crenel_router_bit_radius {:0.3f}".format(c['crenel_width'], c['crenel_rbr']))
+      six.print_(("ERR308: Error, crenel_width {:0.3f} is too small compare to crenel_router_bit_radius {:0.3f}".format(c['crenel_width'], c['crenel_rbr'])))
       sys.exit(2)
 
   ### crenel preparation
@@ -341,7 +342,7 @@ def marked_circle_crenel(ai_x, ai_y, ai_radius, ai_orientation, ai_router_bit_ra
     #r_mcc = cnc25d_api.cnc_cut_outline(cnc25d_api.outline_rotate(A_mcc, ai_x, ai_y, ai_orientation), "marked_circle_crenel")
     r_mcc = cnc25d_api.outline_rotate(A_mcc, ai_x, ai_y, ai_orientation)
   else:
-    print("ERR182: Error, ai_mark_type {:d} doesn't exist".format(ai_mark_type))
+    six.print_(("ERR182: Error, ai_mark_type {:d} doesn't exist".format(ai_mark_type)))
     sys.exit(2)
   return(r_mcc)
     
@@ -442,7 +443,7 @@ def gearwheel_2d_construction(c):
     wh_angle = 2*math.pi/c['wheel_hollow_leg_number']
     wh_leg_top_angle1 = math.asin(float(c['wheel_hollow_leg_width']/2.0+c['wheel_hollow_rbr'])/(c['wheel_hollow_external_radius']-c['wheel_hollow_rbr']))
     if(wh_angle<2*wh_leg_top_angle1+radian_epsilon):
-      print("ERR664: Error, wh_angle {:0.2f} too small compare to wh_leg_top_angle1 {:0.2f}!".format(wh_angle, wh_leg_top_angle1))
+      six.print_(("ERR664: Error, wh_angle {:0.2f} too small compare to wh_leg_top_angle1 {:0.2f}!".format(wh_angle, wh_leg_top_angle1)))
       sys.exit(2)
     wh_leg_bottom_angle1 = math.asin(float(c['wheel_hollow_leg_width']/2.0+c['wheel_hollow_rbr'])/(c['wheel_hollow_internal_radius']+c['wheel_hollow_rbr']))
     #wh_leg_top_angle2 = math.asin((c['wheel_hollow_leg_width']/2)/c['wheel_hollow_external_radius'])
diff --git a/cnc25d/gimbal.py b/cnc25d/gimbal.py
index e841af6..2125e41 100644
--- a/cnc25d/gimbal.py
+++ b/cnc25d/gimbal.py
@@ -29,7 +29,8 @@
 # header for Python / FreeCAD compatibility
 ################################################################
 
-import cnc25d_api
+from . import cnc25d_api
+import six
 cnc25d_api.importing_freecad()
 
 #print("FreeCAD.Version:", FreeCAD.Version())
@@ -52,9 +53,9 @@
 #import svgwrite
 #from dxfwrite import DXFEngine
 # cnc25d
-import bell_bagel_assembly
-import cross_cube
-import angles
+from . import bell_bagel_assembly
+from . import cross_cube
+from . import angles
 
 
 ################################################################
@@ -110,7 +111,7 @@ def gimbal_constraint_check(c):
   ###
   if((c['bottom_angle']!=0)or(c['top_angle']!=0)):
     if((c['pan_angle']!=0)or(c['tilt_angle']!=0)):
-      print("ERR145: Error, roll-pitch angles {:0.3f} {:0.3f} and pan-tilt angles {:0.3f} {:0.3f} are set together".format(c['bottom_angle'], c['top_angle'], c['pan_angle'], c['tilt_angle']))
+      six.print_(("ERR145: Error, roll-pitch angles {:0.3f} {:0.3f} and pan-tilt angles {:0.3f} {:0.3f} are set together".format(c['bottom_angle'], c['top_angle'], c['pan_angle'], c['tilt_angle'])))
       sys.exit(2)
   if((c['pan_angle']!=0)or(c['tilt_angle']!=0)):
     (a1, a2) = angles.pan_tilt_to_roll_pitch(c['pan_angle'], c['tilt_angle'])
@@ -119,24 +120,24 @@ def gimbal_constraint_check(c):
   (b1, b2) = angles.roll_pitch_to_pan_tilt(c['bottom_angle'], c['top_angle'])
   (a1, a2) = angles.pan_tilt_to_roll_pitch(b1, b2)
   if((abs(c['bottom_angle']-a1)>radian_epsilon)or(abs(c['top_angle']-a2)>radian_epsilon)):
-    print("ERR154: Internal error in angle conversion: a1 {:0.3f} {:0.3f}  a2 {:0.3f} {:0.3f}".format(c['bottom_angle'], a1, c['top_angle'], a2))
+    six.print_(("ERR154: Internal error in angle conversion: a1 {:0.3f} {:0.3f}  a2 {:0.3f} {:0.3f}".format(c['bottom_angle'], a1, c['top_angle'], a2)))
     sys.exit(2)
   c['b3'] = angles.roll_pitch_pan_tilt_drift_angle(c['bottom_angle'], c['top_angle'])
   #
   if(abs(c['bottom_angle'])>0.7*math.pi):
-    print("ERR133: Error, bottom_angle {:0.3f} absolute value is too big".format(c['bottom_angle']))
+    six.print_(("ERR133: Error, bottom_angle {:0.3f} absolute value is too big".format(c['bottom_angle'])))
     sys.exit(2)
   if(abs(c['top_angle'])>0.7*math.pi):
-    print("ERR136: Error, top_angle {:0.3f} absolute value is too big".format(c['top_angle']))
+    six.print_(("ERR136: Error, top_angle {:0.3f} absolute value is too big".format(c['top_angle'])))
     sys.exit(2)
   if(abs(c['bottom_angle'])+abs(c['top_angle'])>1.1*math.pi):
-    print("ERR139: Error, bottom_angle {:0.3f} and top_angle {:0.3f} absolute values can not be so large at the same time".format(c['bottom_angle'], c['top_angle']))
+    six.print_(("ERR139: Error, bottom_angle {:0.3f} and top_angle {:0.3f} absolute values can not be so large at the same time".format(c['bottom_angle'], c['top_angle'])))
     sys.exit(2)
   # pan-tilt
   c['pan_angle'] = b1
   c['tilt_angle'] = b2
   if(c['tilt_angle']>0.6*math.pi):
-    print("ERR147: Error, tilt_angle {:0.3f} is to big".format(c['tilt_angle']))
+    six.print_(("ERR147: Error, tilt_angle {:0.3f} is to big".format(c['tilt_angle'])))
     sys.exit(2)
   return(c)
 
diff --git a/cnc25d/importing_freecad.py b/cnc25d/importing_freecad.py
index ea5f893..6cbb064 100644
--- a/cnc25d/importing_freecad.py
+++ b/cnc25d/importing_freecad.py
@@ -38,6 +38,7 @@
 ################################################################
 
 import sys, os
+import six
 
 ################################################################
 # function
@@ -61,7 +62,7 @@ def importing_freecad():
         freecad_path=p
     if(freecad_path==''):
       print("ERR070: Error, the FreeCAD library path has not been found!")
-      print("Add the path of the directory containing FreeCAD.so to the variable FREECADPATH in the file {:s}".format(__FILE__))
+      six.print_(("Add the path of the directory containing FreeCAD.so to the variable FREECADPATH in the file {:s}".format(__FILE__)))
       sys.exit(2)
     #print("dbg101: add FREECADPATH to sys.path")
     sys.path.append(freecad_path)
@@ -77,6 +78,6 @@ def importing_freecad():
 
 if(__name__ == "__main__"):
   importing_freecad()
-  print("FreeCAD.Version:", FreeCAD.Version())
+  six.print_(("FreeCAD.Version:", FreeCAD.Version()))
   FreeCAD.Console.PrintMessage("Hello from PrintMessage!\n")
 
diff --git a/cnc25d/low_torque_transmission.py b/cnc25d/low_torque_transmission.py
index 0adc966..923bd14 100644
--- a/cnc25d/low_torque_transmission.py
+++ b/cnc25d/low_torque_transmission.py
@@ -29,7 +29,8 @@
 # header for Python / FreeCAD compatibility
 ################################################################
 
-import cnc25d_api
+from . import cnc25d_api
+import six
 cnc25d_api.importing_freecad()
 
 #print("FreeCAD.Version:", FreeCAD.Version())
@@ -52,9 +53,9 @@
 #import svgwrite
 #from dxfwrite import DXFEngine
 # cnc25d
-import gearring
-import gearwheel
-import gear_profile # to get the high-level parameter to find the angle position
+from . import gearring
+from . import gearwheel
+from . import gear_profile # to get the high-level parameter to find the angle position
 #import axle_lid
 
 ################################################################
@@ -314,15 +315,15 @@ def ltt_constraint_check(c):
   if(c['planet_nb']==0):
     c['planet_nb'] = c['planet_number_max']
   if(c['planet_nb']>c['planet_number_max']):
-    print("ERR247: Error, planet_nb {:d} is bigger than planet_number_max {:d}".format(c['planet_nb'], c['planet_number_max']))
+    six.print_(("ERR247: Error, planet_nb {:d} is bigger than planet_number_max {:d}".format(c['planet_nb'], c['planet_number_max'])))
     sys.exit(2)
   if(c['planet_nb']<1):
-    print("ERR251: Error, planet_nb {:d} must be bigger or equal to 1".format(c['planet_nb']))
+    six.print_(("ERR251: Error, planet_nb {:d} must be bigger or equal to 1".format(c['planet_nb'])))
     sys.exit(2)
   # epicyclic fitting
   epicyclic_tooth_check = (2*(c['sun_gear_tooth_nb'] + c['planet_gear_tooth_nb'])) % c['planet_nb']
   if(epicyclic_tooth_check != 0):
-    print("WARN243: Warning, not respected epicyclic-gearing-tooth-nb relation: 2*(sun_gear_tooth_nb {:d} + planet_gear_tooth_nb {:d}) % planet_nb {:d} == {:d} (should be 0)".format(c['sun_gear_tooth_nb'], c['planet_gear_tooth_nb'], c['planet_nb'], epicyclic_tooth_check))
+    six.print_(("WARN243: Warning, not respected epicyclic-gearing-tooth-nb relation: 2*(sun_gear_tooth_nb {:d} + planet_gear_tooth_nb {:d}) % planet_nb {:d} == {:d} (should be 0)".format(c['sun_gear_tooth_nb'], c['planet_gear_tooth_nb'], c['planet_nb'], epicyclic_tooth_check)))
   c['planet_angle_inc'] = 2*math.pi/c['planet_nb']
   c['planet_circle_radius'] = (c['sun_gear_tooth_nb'] + c['planet_gear_tooth_nb'])*c['gear_module']/2.0
   c['planet_angle_position'] = []
@@ -355,15 +356,15 @@ def ltt_constraint_check(c):
     c['sun_oriantation_angles'].append(i_gp.get_constraint()['second_positive_initial_angle'])
     #c['sun_oriantation_angles'].append(math.fmod(i_gp.get_constraint()['second_positive_initial_angle']+sun_half_tooth_angle, 2*sun_half_tooth_angle)-sun_half_tooth_angle) # it is better to do this normaization when comparing
     if(abs(sun_x_position - 0.0)>radian_epsilon):
-      print("ERR286: Error, sun_x_position {:0.3f} is not at the expected coordinate 0.0".format(sun_x_position))
+      six.print_(("ERR286: Error, sun_x_position {:0.3f} is not at the expected coordinate 0.0".format(sun_x_position)))
       sys.exit(2)
     if(abs(sun_y_position - 0.0)>radian_epsilon):
-      print("ERR289: Error, sun_y_position {:0.3f} is not at the expected coordinate 0.0".format(sun_y_position))
+      six.print_(("ERR289: Error, sun_y_position {:0.3f} is not at the expected coordinate 0.0".format(sun_y_position)))
       sys.exit(2)
     #if(abs(c['sun_oriantation_angles'][i]-c['sun_oriantation_angles'][0])>radian_epsilon):
     angle_diff = math.fmod(c['sun_oriantation_angles'][i]-c['sun_oriantation_angles'][0]+11*sun_half_tooth_angle, 2*sun_half_tooth_angle)-sun_half_tooth_angle # might habe issue when the difference is negative
     if(abs(angle_diff)>radian_epsilon):
-      print("ERR293: Error, {:d}-sun_oriantation_angles {:0.3f} is different from 0-sun_oriantation_angles {:0.3f}".format(i, c['sun_oriantation_angles'][i], c['sun_oriantation_angles'][0]))
+      six.print_(("ERR293: Error, {:d}-sun_oriantation_angles {:0.3f} is different from 0-sun_oriantation_angles {:0.3f}".format(i, c['sun_oriantation_angles'][i], c['sun_oriantation_angles'][0])))
       #print("dbg306: sun_half_tooth_angle:", sun_half_tooth_angle, 2*sun_half_tooth_angle, i_gp.get_constraint()['second_pi_module_angle'])
       #print("dbg307: sun_oriantation_angles:", c['sun_oriantation_angles'])
       #print("dbg369: angle_diff:", angle_diff)
@@ -374,24 +375,24 @@ def ltt_constraint_check(c):
   c['sun_axle_radius'] = c['sun_axle_diameter']/2.0
   c['sun_gear_hollow_diameter'] = c['gear_module']*(c['sun_gear_tooth_nb']-3)
   if(c['sun_axle_diameter']+2*c['sun_spacer_length']>c['sun_gear_hollow_diameter']):
-    print("ERR310: Error, sun_axle_diameter {:0.3f} or sun_spacer_length {:0.3f} are too big compare to sun_gear_hollow_diameter {:0.3f}".format(c['sun_axle_diameter'], c['sun_spacer_length'], c['sun_gear_hollow_diameter']))
+    six.print_(("ERR310: Error, sun_axle_diameter {:0.3f} or sun_spacer_length {:0.3f} are too big compare to sun_gear_hollow_diameter {:0.3f}".format(c['sun_axle_diameter'], c['sun_spacer_length'], c['sun_gear_hollow_diameter'])))
     sys.exit(2)
   # planet axle and spacer
   c['planet_axle_radius'] = c['planet_axle_diameter']/2.0
   c['planet_gear_hollow_diameter'] = c['gear_module']*(c['planet_gear_tooth_nb']-3)
   if(c['planet_axle_diameter']+2*c['planet_spacer_length']>c['planet_gear_hollow_diameter']):
-    print("ERR311: Error, planet_axle_diameter {:0.3f} or planet_spacer_length {:0.3f} are too big compare to planet_gear_hollow_diameter {:0.3f}".format(c['planet_axle_diameter'], c['planet_spacer_length'], c['planet_gear_hollow_diameter']))
+    six.print_(("ERR311: Error, planet_axle_diameter {:0.3f} or planet_spacer_length {:0.3f} are too big compare to planet_gear_hollow_diameter {:0.3f}".format(c['planet_axle_diameter'], c['planet_spacer_length'], c['planet_gear_hollow_diameter'])))
     sys.exit(2)
   # planet_carrier axle and spacer
   c['planet_carrier_axle_radius'] = c['planet_carrier_axle_diameter']/2.0
   if(c['planet_carrier_axle_diameter']+2*c['planet_carrier_spacer_length']>c['planet_gear_hollow_diameter']):
-    print("ERR315: Error, planet_carrier_axle_diameter {:0.3f} or planet_carrier_spacer_length {:0.3f} are too big compare to planet_gear_hollow_diameter {:0.3f}".format(c['planet_carrier_axle_diameter'], c['planet_carrier_spacer_length'], c['planet_gear_hollow_diameter']))
+    six.print_(("ERR315: Error, planet_carrier_axle_diameter {:0.3f} or planet_carrier_spacer_length {:0.3f} are too big compare to planet_gear_hollow_diameter {:0.3f}".format(c['planet_carrier_axle_diameter'], c['planet_carrier_spacer_length'], c['planet_gear_hollow_diameter'])))
     sys.exit(2)
   if(c['planet_carrier_axle_holder_diameter']>c['planet_gear_hollow_diameter']):
-    print("ERR318: Error, planet_carrier_axle_holder_diameter {:0.3f} is too big compare to planet_gear_hollow_diameter {:0.3f}".format(c['planet_carrier_axle_holder_diameter'], c['planet_gear_hollow_diameter']))
+    six.print_(("ERR318: Error, planet_carrier_axle_holder_diameter {:0.3f} is too big compare to planet_gear_hollow_diameter {:0.3f}".format(c['planet_carrier_axle_holder_diameter'], c['planet_gear_hollow_diameter'])))
     sys.exit(2)
   if(c['planet_axle_diameter']+2*c['planet_spacer_length']>c['planet_carrier_axle_holder_diameter']):
-    print("ERR321: Error, planet_carrier_axle_holder_diameter {:0.3f} is too small compare to planet_axle_diameter {:0.3f} and planet_spacer_length {:0.3f}".format(c['planet_axle_diameter'], c['planet_spacer_length'], c['planet_carrier_axle_holder_diameter']))
+    six.print_(("ERR321: Error, planet_carrier_axle_holder_diameter {:0.3f} is too small compare to planet_axle_diameter {:0.3f} and planet_spacer_length {:0.3f}".format(c['planet_axle_diameter'], c['planet_spacer_length'], c['planet_carrier_axle_holder_diameter'])))
     sys.exit(2)
   c['planet_carrier_axle_holder_radius'] = c['planet_carrier_axle_holder_diameter']/2.0
   # planet_carrier_external_diameter
@@ -400,10 +401,10 @@ def ltt_constraint_check(c):
     c['planet_carrier_external_diameter'] = c['planet_carrier_external_diameter_max']
   c['planet_carrier_external_radius'] = c['planet_carrier_external_diameter']/2.0
   if(c['planet_carrier_external_diameter']>c['planet_carrier_external_diameter_max']):
-    print("ERR329: Error, planet_carrier_external_diameter {:0.3f} is bigger than planet_carrier_external_diameter_max {:0.3f}".format(c['planet_carrier_external_diameter'], c['planet_carrier_external_diameter_max']))
+    six.print_(("ERR329: Error, planet_carrier_external_diameter {:0.3f} is bigger than planet_carrier_external_diameter_max {:0.3f}".format(c['planet_carrier_external_diameter'], c['planet_carrier_external_diameter_max'])))
     sys.exit(2)
   if(c['planet_carrier_external_radius']<c['planet_circle_radius']-c['planet_carrier_axle_holder_radius']+radian_epsilon):
-    print("ERR325: Error, planet_carrier_external_radius {:0.3f} is too small compare to planet_circle_radius {:0.3f} and planet_carrier_axle_holder_radius {:0.3f}".format(c['planet_carrier_external_radius'], c['planet_circle_radius'], c['planet_carrier_axle_holder_radius']))
+    six.print_(("ERR325: Error, planet_carrier_external_radius {:0.3f} is too small compare to planet_circle_radius {:0.3f} and planet_carrier_axle_holder_radius {:0.3f}".format(c['planet_carrier_external_radius'], c['planet_circle_radius'], c['planet_carrier_axle_holder_radius'])))
     sys.exit(2)
   # planet_carrier_internal_diameter
   c['planet_carrier_internal_diameter_min'] = c['gear_module']*(c['sun_gear_tooth_nb']+4)
@@ -412,16 +413,16 @@ def ltt_constraint_check(c):
     c['planet_carrier_internal_diameter'] = c['planet_carrier_internal_diameter_default']
   c['planet_carrier_internal_radius'] = c['planet_carrier_internal_diameter']/2.0
   if(c['planet_carrier_internal_diameter']<c['planet_carrier_internal_diameter_min']):
-    print("ERR342: Error, planet_carrier_internal_diameter {:0.3f} is smaller than planet_carrier_internal_diameter_min {:0.3f}".format(c['planet_carrier_internal_diameter'], c['planet_carrier_internal_diameter_min']))
+    six.print_(("ERR342: Error, planet_carrier_internal_diameter {:0.3f} is smaller than planet_carrier_internal_diameter_min {:0.3f}".format(c['planet_carrier_internal_diameter'], c['planet_carrier_internal_diameter_min'])))
     sys.exit(2)
   if(c['planet_carrier_internal_diameter']>c['planet_carrier_external_diameter']-radian_epsilon):
-    print("ERR345: Error, planet_carrier_internal_diameter {:0.3f} is bigger than planet_carrier_external_diameter {:0.3f}".format(c['planet_carrier_internal_diameter'], c['planet_carrier_external_diameter']))
+    six.print_(("ERR345: Error, planet_carrier_internal_diameter {:0.3f} is bigger than planet_carrier_external_diameter {:0.3f}".format(c['planet_carrier_internal_diameter'], c['planet_carrier_external_diameter'])))
     sys.exit(2)
   # planet_carrier_rear_smooth_radius
   if(c['planet_carrier_rear_smooth_radius']<c['cnc_router_bit_radius']):
     c['planet_carrier_rear_smooth_radius'] = c['cnc_router_bit_radius']
   if(c['planet_carrier_rear_smooth_radius']>min(c['planet_carrier_internal_radius'], c['planet_carrier_axle_holder_radius'])):
-    print("ERR351: Error, planet_carrier_rear_smooth_radius {:0.3f} is too big compare to planet_carrier_internal_radius {:0.3f} and planet_carrier_axle_holder_radius {:0.3f}".format(c['planet_carrier_rear_smooth_radius'], c['planet_carrier_internal_radius'],  c['planet_carrier_axle_holder_radius']))
+    six.print_(("ERR351: Error, planet_carrier_rear_smooth_radius {:0.3f} is too big compare to planet_carrier_internal_radius {:0.3f} and planet_carrier_axle_holder_radius {:0.3f}".format(c['planet_carrier_rear_smooth_radius'], c['planet_carrier_internal_radius'],  c['planet_carrier_axle_holder_radius'])))
     sys.exit(2)
   # planet_carrier_middle_clearance_diameter
   c['planet_carrier_middle_clearance_diameter_min'] = c['gear_module']*(c['planet_gear_tooth_nb']+4)
@@ -429,7 +430,7 @@ def ltt_constraint_check(c):
     c['planet_carrier_middle_clearance_diameter'] = c['planet_carrier_middle_clearance_diameter_min']
   c['planet_carrier_middle_clearance_radius'] = c['planet_carrier_middle_clearance_diameter']/2.0
   if(c['planet_carrier_middle_clearance_diameter']<c['planet_carrier_middle_clearance_diameter_min']):
-    print("ERR370: Error, planet_carrier_middle_clearance_diameter {:0.3f} is smaller than planet_carrier_middle_clearance_diameter_min {:0.3f}".format(c['planet_carrier_middle_clearance_diameter'], c['planet_carrier_middle_clearance_diameter_min']))
+    six.print_(("ERR370: Error, planet_carrier_middle_clearance_diameter {:0.3f} is smaller than planet_carrier_middle_clearance_diameter_min {:0.3f}".format(c['planet_carrier_middle_clearance_diameter'], c['planet_carrier_middle_clearance_diameter_min'])))
     sys.exit(2)
   # planet_carrier_middle_smooth_radius
   if(c['planet_carrier_middle_smooth_radius']<c['cnc_router_bit_radius']):
@@ -458,7 +459,7 @@ def ltt_constraint_check(c):
     aOAD = math.pi/2 - c['planet_angle_inc']/2.0
     aOAF = aOAD + aDAE
     if(aOAG<aOAF):
-      print("ERR395: Error, planet_carrier_external_radius {:0.3f} is too small compare to planet_carrier_middle_clearance_radius {:0.3f} or planet_carrier_middle_smooth_radius {:0.3f}".format(c['planet_carrier_external_radius'], c['planet_carrier_middle_clearance_radius'], c['planet_carrier_middle_smooth_radius']))
+      six.print_(("ERR395: Error, planet_carrier_external_radius {:0.3f} is too small compare to planet_carrier_middle_clearance_radius {:0.3f} or planet_carrier_middle_smooth_radius {:0.3f}".format(c['planet_carrier_external_radius'], c['planet_carrier_middle_clearance_radius'], c['planet_carrier_middle_smooth_radius'])))
       sys.exit(2)
     elif(aOAH<aOAF):
       c['middle_planet_carrier_aOAH'] = aOAF # a bit more to ensure a good cnc_cut
@@ -514,7 +515,7 @@ def sub_fitting_square(ai_r, ai_l1, ai_l2):
     c['middle_planet_carrier_fitting_square_int_angle'] = middle_fs['int_angle']
     c['middle_planet_carrier_fitting_square_int_radius'] = middle_fs['int_radius']
     if(middle_fs['ref_radius']<c['middle_planet_carrier_internal_radius']):
-      print("ERR435: Error, planet_carrier_fitting_square_l2 {:0.3f} is too big".format(c['planet_carrier_fitting_square_l2']))
+      six.print_(("ERR435: Error, planet_carrier_fitting_square_l2 {:0.3f} is too big".format(c['planet_carrier_fitting_square_l2'])))
       sys.exit(2)
     c['planet_carrier_fitting_hole_ref'] = middle_fs['ref_radius']
     front_fs = sub_fitting_square(c['planet_carrier_external_radius'],
@@ -533,7 +534,7 @@ def sub_fitting_square(ai_r, ai_l1, ai_l2):
     c['planet_carrier_fitting_hole_position_angle'] = aBOC
   # step_nb
   if(c['step_nb']<1):
-    print("ERR456: Error, step_nb {:d} is bigger or equal to 1".format(c['step_nb']))
+    six.print_(("ERR456: Error, step_nb {:d} is bigger or equal to 1".format(c['step_nb'])))
     sys.exit(2)
   # first step z-dimension
   c['middle_planet_carrier_width'] = c['rear_planet_carrier_spacer_width'] + c['planet_slack'] + c['planet_width'] + c['planet_spacer_width']
@@ -548,43 +549,43 @@ def sub_fitting_square(ai_r, ai_l1, ai_l2):
   # hexagon_hole_diameter
   c['hexagon_hole_radius'] = c['hexagon_hole_diameter']/2.0
   if(c['hexagon_hole_radius']<c['sun_axle_radius']):
-    print("ERR472: Error, hexagon_hole_radius {:0.3f} is smaller than sun_axle_radius {:0.3f}".format(c['hexagon_hole_radius'], c['sun_axle_radius']))
+    six.print_(("ERR472: Error, hexagon_hole_radius {:0.3f} is smaller than sun_axle_radius {:0.3f}".format(c['hexagon_hole_radius'], c['sun_axle_radius'])))
     sys.exit(2)
   if(c['hexagon_length']<c['hexagon_hole_diameter']+radian_epsilon):
-    print("ERR475: Error, hexagon_length {:0.3f} must be bigger than hexagon_hole_diameter {:0.3f}".format(c['hexagon_length'], c['hexagon_hole_diameter']))
+    six.print_(("ERR475: Error, hexagon_length {:0.3f} must be bigger than hexagon_hole_diameter {:0.3f}".format(c['hexagon_length'], c['hexagon_hole_diameter'])))
     sys.exit(2)
   c['hexagon_angle'] = math.pi/6
   c['hexagon_radius'] = c['hexagon_length']/2.0/math.cos(c['hexagon_angle'])
   if(c['hexagon_radius']>c['planet_carrier_external_radius']-radian_epsilon):
-    print("ERR480: Error, hexagon_length {:0.3f} is too big compare to planet_carrier_external_diameter {:0.3f}".format(c['hexagon_length'], c['planet_carrier_external_diameter']))
+    six.print_(("ERR480: Error, hexagon_length {:0.3f} is too big compare to planet_carrier_external_diameter {:0.3f}".format(c['hexagon_length'], c['planet_carrier_external_diameter'])))
     sys.exit(2)
   # hexagon_smooth_radius
   if(c['hexagon_smooth_radius']<c['cnc_router_bit_radius']):
     c['hexagon_smooth_radius'] = c['cnc_router_bit_radius']
   if(c['hexagon_smooth_radius']>c['hexagon_length']/2.0-radian_epsilon):
-    print("ERR486: Error, hexagon_smooth_radius {:0.3f} is too big compare to hexagon_length {:0.3f}".format(c['hexagon_smooth_radius'], c['hexagon_length']))
+    six.print_(("ERR486: Error, hexagon_smooth_radius {:0.3f} is too big compare to hexagon_length {:0.3f}".format(c['hexagon_smooth_radius'], c['hexagon_length'])))
     sys.exit(2)
   # output_cover_width, output_holder_width, hexagon_width
   c['output_holder_width2'] = c['output_holder_width'] + c['output_cover_width']
   c['output_cover_depth'] = c['hexagon_width'] - c['output_holder_width']
   if(c['output_cover_depth']<0):
-    print("ERR494: Error, output_cover_depth {:0.3f} < 0. output_cover_width{:0.3f} or hexagon_width {:0.3f} too small or output_holder_width {:0.3f} too big".format(c['output_cover_depth'], c['output_cover_width'], c['hexagon_width'], c['output_holder_width']))
+    six.print_(("ERR494: Error, output_cover_depth {:0.3f} < 0. output_cover_width{:0.3f} or hexagon_width {:0.3f} too small or output_holder_width {:0.3f} too big".format(c['output_cover_depth'], c['output_cover_width'], c['hexagon_width'], c['output_holder_width'])))
     sys.exit(2)
   if(c['output_cover_depth']>c['output_cover_width']):
-    print("ERR497: Error, output_cover_depth {:0.3f} is too big compare to output_cover_width {:0.3f}".format(c['output_cover_depth'], c['output_cover_width']))
+    six.print_(("ERR497: Error, output_cover_depth {:0.3f} is too big compare to output_cover_width {:0.3f}".format(c['output_cover_depth'], c['output_cover_width'])))
     sys.exit(2)
   if(c['output_cover_depth']>c['input_slack']):
-    print("ERR500: Error, input_slack {:0.3f} is too small compare to output_cover_depth {:0.3f}".format(c['input_slack'], c['output_cover_depth']))
+    six.print_(("ERR500: Error, input_slack {:0.3f} is too small compare to output_cover_depth {:0.3f}".format(c['input_slack'], c['output_cover_depth'])))
     sys.exit(2)
   # output_cover_radius_slack
   c['holder_radius'] = i_gr.get_constraint()['holder_radius']
   c['output_cover_radius'] = c['planet_carrier_external_radius']+c['output_cover_radius_slack']
   if(c['output_cover_radius']>c['holder_radius']-radian_epsilon):
-    print("ERR508: Error, output_cover_radius_slack {:0.3f} is too big compare to planet_carrier_external_radius {:0.3f} and holder_radius {:0.3f}".format(c['output_cover_radius_slack'], c['planet_carrier_external_radius'], c['holder_radius']))
+    six.print_(("ERR508: Error, output_cover_radius_slack {:0.3f} is too big compare to planet_carrier_external_radius {:0.3f} and holder_radius {:0.3f}".format(c['output_cover_radius_slack'], c['planet_carrier_external_radius'], c['holder_radius'])))
     sys.exit(2)
   c['output_holder_radius'] = c['holder_radius'] - c['output_holder_thickness']
   if(c['output_holder_radius']<c['output_cover_radius']):
-    print("ERR563: Error, output_holder_thickness {:0.3} or output_cover_radius_slack {:0.3} are too big".format(c['output_holder_thickness'], c['output_cover_radius_slack']))
+    six.print_(("ERR563: Error, output_holder_thickness {:0.3} or output_cover_radius_slack {:0.3} are too big".format(c['output_holder_thickness'], c['output_cover_radius_slack'])))
     sys.exit(2)
   c['output_holder_crenel_nb_default'] = int((c['holder_crenel_number']+1)/2)+1
   if(c['output_holder_crenel_nb']==0):
@@ -592,13 +593,13 @@ def sub_fitting_square(ai_r, ai_l1, ai_l2):
   # output_axle_holder
   c['output_axle_radius'] = c['output_axle_diameter']/2.0
   if(c['output_axle_radius']<radian_epsilon):
-    print("ERR513: Error, output_axle_diameter {:0.3f} is too small".format(c['output_axle_diameter']))
+    six.print_(("ERR513: Error, output_axle_diameter {:0.3f} is too small".format(c['output_axle_diameter'])))
     sys.exit(2)
   if(c['axle_holder_D']<c['output_axle_diameter']-radian_epsilon):
-    print("ERR516: Error, axle_holder_D {:0.3f} is too small compare to output_axle_diameter {:0.3f}".format(c['axle_holder_D'], c['output_axle_diameter']))
+    six.print_(("ERR516: Error, axle_holder_D {:0.3f} is too small compare to output_axle_diameter {:0.3f}".format(c['axle_holder_D'], c['output_axle_diameter'])))
     sys.exit(2)
   if(c['axle_holder_D']/2.0+c['axle_holder_B']>c['holder_radius']):
-    print("ERR519: Error, axle_holder_D {:0.3f} or axle_holder_B {:0.3f} are too big compare to holder_radius {:0.3f}".format(c['axle_holder_D'], c['axle_holder_B'], c['holder_radius']))
+    six.print_(("ERR519: Error, axle_holder_D {:0.3f} or axle_holder_B {:0.3f} are too big compare to holder_radius {:0.3f}".format(c['axle_holder_D'], c['axle_holder_B'], c['holder_radius'])))
     sys.exit(2)
   c['output_axle_cylinder_width'] = c['axle_holder_A'] + c['axle_holder_C']
   c['output_axle_cylinder_thickness'] = c['axle_holder_D']/2.0 - c['output_axle_radius']
@@ -608,33 +609,33 @@ def sub_fitting_square(ai_r, ai_l1, ai_l2):
   # input_axle_diameter
   c['input_axle_radius'] = c['input_axle_diameter']/2.0
   if(c['input_axle_radius']>c['sun_axle_radius']):
-    print("ERR521: Error, input_axle_radius {:0.3f} is too big compare to sun_axle_radius {:0.3f}".format(c['input_axle_radius'], c['sun_axle_radius']))
+    six.print_(("ERR521: Error, input_axle_radius {:0.3f} is too big compare to sun_axle_radius {:0.3f}".format(c['input_axle_radius'], c['sun_axle_radius'])))
     sys.exit(2)
   # input_sun_width
   c['input_sun_width_min'] = c['sun_width']
   if(c['input_sun_width']==0):
     c['input_sun_width'] = c['input_sun_width_min']
   if(c['input_sun_width']<c['input_sun_width_min']):
-    print("ERR525: Error, input_sun_width {:0.3f} is too small compare to input_sun_width_min {:0.3f}".format(c['input_sun_width'], c['input_sun_width_min']))
+    six.print_(("ERR525: Error, input_sun_width {:0.3f} is too small compare to input_sun_width_min {:0.3f}".format(c['input_sun_width'], c['input_sun_width_min'])))
     sys.exit(2)
   # motor_holder
   c['motor_shape_rectangle_ncircle'] = True
   if(c['motor_y_width']==0):
     c['motor_shape_rectangle_ncircle'] = False
   if(c['motor_x_width']<c['gear_module']*(c['sun_gear_tooth_nb']+2)):
-    print("WARN532: Warning, motor_x_width {:0.3f} is small compare to the sun-gear".format(c['motor_x_width']))
+    six.print_(("WARN532: Warning, motor_x_width {:0.3f} is small compare to the sun-gear".format(c['motor_x_width'])))
   if(c['motor_shape_rectangle_ncircle']):
     if(c['motor_y_width']<c['gear_module']*(c['sun_gear_tooth_nb']+2)):
-      print("WARN535: Warning, motor_y_width {:0.3f} is small compare to the sun-gear".format(c['motor_y_width']))
+      six.print_(("WARN535: Warning, motor_y_width {:0.3f} is small compare to the sun-gear".format(c['motor_y_width'])))
   motor_xy_width_max = max(c['motor_x_width'], c['motor_y_width'])
   if((motor_xy_width_max/2.0+c['motor_holder_E']+c['motor_holder_B'])>c['holder_radius']):
-    print("ERR541: Error, motor_holder_E {:0.3f} or motor_holder_B {:0.3f} are too big compare to holder_radius {:0.3f}".format(c['motor_holder_E'], c['motor_holder_B'], c['holder_radius']))
+    six.print_(("ERR541: Error, motor_holder_E {:0.3f} or motor_holder_B {:0.3f} are too big compare to holder_radius {:0.3f}".format(c['motor_holder_E'], c['motor_holder_B'], c['holder_radius'])))
     sys.exit(2)
   motor_xy_width_min = c['motor_x_width']
   if(c['motor_shape_rectangle_ncircle']):
     motor_xy_width_min = min(c['motor_x_width'], c['motor_y_width'])
   if(c['motor_holder_leg_width']>motor_xy_width_min/4.0):
-    print("ERR545: Error, motor_holder_leg_width {:0.3f} is too big compare to motor_xy_width_min {:0.3f}".format(c['motor_holder_leg_width'], motor_xy_width_min))
+    six.print_(("ERR545: Error, motor_holder_leg_width {:0.3f} is too big compare to motor_xy_width_min {:0.3f}".format(c['motor_holder_leg_width'], motor_xy_width_min)))
     sys.exit(2)
   c['motor_holder_leg_x'] = c['motor_holder_A'] + c['motor_holder_C'] + c['motor_holder_D']
   c['motor_holder_leg_y'] = c['motor_holder_B'] + c['motor_holder_E']
diff --git a/cnc25d/motor_lid.py b/cnc25d/motor_lid.py
index 81750d8..cd2dfbb 100644
--- a/cnc25d/motor_lid.py
+++ b/cnc25d/motor_lid.py
@@ -29,7 +29,8 @@
 # header for Python / FreeCAD compatibility
 ################################################################
 
-import cnc25d_api
+from . import cnc25d_api
+import six
 cnc25d_api.importing_freecad()
 
 #print("FreeCAD.Version:", FreeCAD.Version())
@@ -52,7 +53,7 @@
 #import svgwrite
 #from dxfwrite import DXFEngine
 # cnc25d
-import axle_lid
+from . import axle_lid
 
 
 ################################################################
@@ -190,11 +191,11 @@ def motor_lid_constraint_check(c):
   if(c['smoothing_radius']==0):
     c['smoothing_radius'] = c['cnc_router_bit_radius']
   if(c['smoothing_radius']<c['cnc_router_bit_radius']):
-    print("ERR232: Error, smoothing_radius {:0.3f} is smaller than cnc_router_bit_radius {:0.3f}".format(c['smoothing_radius'], c['cnc_router_bit_radius']))
+    six.print_(("ERR232: Error, smoothing_radius {:0.3f} is smaller than cnc_router_bit_radius {:0.3f}".format(c['smoothing_radius'], c['cnc_router_bit_radius'])))
     sys.exit(2)
   ## holder-axis
   if((c['axle_B_place']!='small')and(c['axle_B_place']!='large')):
-    print("ERR236: Error, axle_B_place {:s} accept only 'small' or 'large'".format(c['axle_B_place']))
+    six.print_(("ERR236: Error, axle_B_place {:s} accept only 'small' or 'large'".format(c['axle_B_place'])))
     sys.exit(2)
   c['holder_crenel_number'] = c['holder_crenel_number']
   middle_crenel_1 = 0
@@ -218,7 +219,7 @@ def motor_lid_constraint_check(c):
     a_cab = math.asin(sin_a)
     a_AC = c['axle_B_angle'] + math.copysign(a_cab, c['axle_C_angle']) # angle (holder-axis, AC)
     if(l_AC<c['holder_radius']):
-      print("ERR245: l_AC {:0.3} is smaller than the holder_radius {:0.3f}".format(l_AC, c['holder_radius']))
+      six.print_(("ERR245: l_AC {:0.3} is smaller than the holder_radius {:0.3f}".format(l_AC, c['holder_radius'])))
       sys.exit(2)
   ### holder_A
   c['holder_A_axle_B_place'] = c['axle_B_place']
@@ -230,7 +231,7 @@ def motor_lid_constraint_check(c):
     c['axle_B_external_radius'] = 2*c['axle_B_radius']
   if(c['holder_A_axle_B_place'] != 'none'):
     if(c['axle_B_external_radius']<c['axle_B_radius']+radian_epsilon):
-      print("ERR262: Error, axle_B_external_radius {:0.3f} is smaller than axle_B_radius {:0.3f}".format(c['axle_B_external_radius'], c['axle_B_radius']))
+      six.print_(("ERR262: Error, axle_B_external_radius {:0.3f} is smaller than axle_B_radius {:0.3f}".format(c['axle_B_external_radius'], c['axle_B_radius'])))
       sys.exit(2)
   i_axle_lid = axle_lid.axle_lid()
   i_axle_lid.apply_external_constraint(holder_A_al2ml(c))
@@ -246,7 +247,7 @@ def motor_lid_constraint_check(c):
     c['axle_C_external_radius'] = 2*c['axle_C_hole_radius']
   if(c['holder_B_axle_B_place'] != 'none'):
     if(c['axle_C_external_radius']<c['axle_C_hole_radius']+radian_epsilon):
-      print("ERR280: Error, axle_C_external_radius {:0.3f} is too small compare to axle_C_hole_radius {:0.3f}".format(c['axle_C_external_radius'], c['axle_C_hole_radius']))
+      six.print_(("ERR280: Error, axle_C_external_radius {:0.3f} is too small compare to axle_C_hole_radius {:0.3f}".format(c['axle_C_external_radius'], c['axle_C_hole_radius'])))
       sys.exit(2)
   c['AC_length'] = l_AC
   c['AC_angle'] = a_AC
@@ -267,19 +268,19 @@ def motor_lid_constraint_check(c):
     c['fastening_BC_external_radius'] = 2*c['fastening_BC_hole_radius']
   if(c['fastening_BC_external_radius']>0):
     if(c['fastening_BC_external_radius']<c['fastening_BC_hole_radius']+radian_epsilon):
-      print("ERR367: Error, fastening_BC_external_radius {:0.3f} is too small compare to fastening_BC_hole_radius {:0.3f}".format(c['fastening_BC_external_radius'], c['fastening_BC_hole_radius']))
+      six.print_(("ERR367: Error, fastening_BC_external_radius {:0.3f} is too small compare to fastening_BC_hole_radius {:0.3f}".format(c['fastening_BC_external_radius'], c['fastening_BC_hole_radius'])))
       sys.exit(2)
     if(c['axle_B_central_radius']<c['axle_B_radius']+radian_epsilon):
-      print("ERR355: Error, axle_B_central_radius {:0.3f} is too small compare to axle_B_radius {:0.3f}".format(c['axle_B_central_radius'], c['axle_B_radius']))
+      six.print_(("ERR355: Error, axle_B_central_radius {:0.3f} is too small compare to axle_B_radius {:0.3f}".format(c['axle_B_central_radius'], c['axle_B_radius'])))
       sys.exit(2)
     if(c['fastening_BC_hole_radius']<radian_epsilon):
-      print("ERR324: Error, fastening_BC_hole_radius {:0.3f} is too small".format(c['fastening_BC_hole_radius']))
+      six.print_(("ERR324: Error, fastening_BC_hole_radius {:0.3f} is too small".format(c['fastening_BC_hole_radius'])))
       sys.exit(2)
     if((c['fastening_BC_external_radius']+c['fastening_BC_hole_radius'])>c['fastening_BC_bottom_position_radius']):
-      print("ERR327: Error, fastening_BC_bottom_position_radius {:0.3f} is too small compare to fastening_BC_external_radius {:0.3f} and fastening_BC_hole_radius {:0.3f}".format(c['fastening_BC_bottom_position_radius'], c['fastening_BC_external_radius'], c['fastening_BC_hole_radius']))
+      six.print_(("ERR327: Error, fastening_BC_bottom_position_radius {:0.3f} is too small compare to fastening_BC_external_radius {:0.3f} and fastening_BC_hole_radius {:0.3f}".format(c['fastening_BC_bottom_position_radius'], c['fastening_BC_external_radius'], c['fastening_BC_hole_radius'])))
       sys.exit(2)
     if((c['fastening_BC_external_radius']+c['fastening_BC_hole_radius'])>c['fastening_BC_top_position_radius']):
-      print("ERR330: Error, fastening_BC_top_position_radius {:0.3f} is too small compare to fastening_BC_external_radius {:0.3f} and fastening_BC_hole_radius {:0.3f}".format(c['fastening_BC_top_position_radius'], c['fastening_BC_external_radius'], c['fastening_BC_hole_radius']))
+      six.print_(("ERR330: Error, fastening_BC_top_position_radius {:0.3f} is too small compare to fastening_BC_external_radius {:0.3f} and fastening_BC_hole_radius {:0.3f}".format(c['fastening_BC_top_position_radius'], c['fastening_BC_external_radius'], c['fastening_BC_hole_radius'])))
       sys.exit(2)
   ###
   return(c)
diff --git a/cnc25d/outline_backends.py b/cnc25d/outline_backends.py
index 995013c..59c4058 100644
--- a/cnc25d/outline_backends.py
+++ b/cnc25d/outline_backends.py
@@ -28,14 +28,15 @@
 # python behavior
 ################################################################
 
-from __future__ import division # to get float division
+ # to get float division
 
 
 ################################################################
 # header for Python / FreeCAD compatibility
 ################################################################
 
-import importing_freecad
+from . import importing_freecad
+import six
 importing_freecad.importing_freecad()
 
 #print("FreeCAD.Version:", FreeCAD.Version())
@@ -52,12 +53,12 @@
 import sys, argparse
 import svgwrite
 from dxfwrite import DXFEngine
-import Tkinter
+from six.moves import tkinter
 import time # for time.sleep to help Tkinter to finish properly
-import display_backend
-import cnc_outline # just used in figure_simple_display() for cnc_outline.outline_rotate, closed(), check_outline_format() and ideal_outline()
-import export_2d # just for test enhancement
-import design_help # just for get_effective_args() and mkdir_p
+from . import display_backend
+from . import cnc_outline # just used in figure_simple_display() for cnc_outline.outline_rotate, closed(), check_outline_format() and ideal_outline()
+from . import export_2d # just for test enhancement
+from . import design_help # just for get_effective_args() and mkdir_p
 
 
 ################################################################
@@ -80,7 +81,7 @@ def complete_circle(ai_center, ai_radius, ai_resolution):
   r_points = []
   # calculation of the angle resolution:
   if(ai_resolution<3):
-    print("ERR821: The ai_resolution is smaller than 3. Current ai_resolution = {:d}".format(ai_resolution))
+    six.print_(("ERR821: The ai_resolution is smaller than 3. Current ai_resolution = {:d}".format(ai_resolution)))
     sys.exit(2)
   #print("dbg424: ai_radius:", ai_radius)
   circle_resolution = int(ai_resolution * ai_radius) # circle resolution increase with the radius
@@ -124,7 +125,7 @@ def arc_3_points_to_radius_center_angles(ai_start, ai_middle, ai_end):
   lbc = math.sqrt((ptcx-ptbx)**2+(ptcy-ptby)**2)
   if(lab<length_epsilon):
     print("ERR811: Error, A and B are almost identical")
-    print("dbg559: pta={:0.2f} {:0.2f}  ptb={:0.2f} {:0.2f}  ptc={:0.2f} {:0.2f}".format(ptax, ptay, ptbx, ptby, ptcx, ptcy))
+    six.print_(("dbg559: pta={:0.2f} {:0.2f}  ptb={:0.2f} {:0.2f}  ptc={:0.2f} {:0.2f}".format(ptax, ptay, ptbx, ptby, ptcx, ptcy)))
     sys.exit(2)
   if(lbc<length_epsilon):
     print("ERR812: Error, B and C are almost identical")
@@ -231,7 +232,7 @@ def arc_of_circle(ai_start, ai_middle, ai_end, ai_resolution):
   ### real arc case
   # calculation of the angle resolution:
   if(ai_resolution<3):
-    print("ERR821: The ai_resolution is smaller than 3. Current ai_resolution = {:d}".format(ai_resolution))
+    six.print_(("ERR821: The ai_resolution is smaller than 3. Current ai_resolution = {:d}".format(ai_resolution)))
     sys.exit(2)
   #print("dbg414: arc radius: lia:", lia)
   circle_resolution = ai_resolution * lia # angle resolution increase with the radius
@@ -268,7 +269,7 @@ def outline_arc_line_with_freecad(ai_segments, ai_outline_closed):
   fc_outline = []
   for i in range(segment_nb-1):
     if((abs(ai_segments[i][-2]-ai_segments[i+1][-2])<length_epsilon)and(abs(ai_segments[i][-1]-ai_segments[i+1][-1])<length_epsilon)):
-      print("ERR264: Error, point of index {:d} and {:d} are identical. x {:0.5f}  y {:0.5f}".format(i,i+1, ai_segments[i][-2], ai_segments[i][-1]))
+      six.print_(("ERR264: Error, point of index {:d} and {:d} are identical. x {:0.5f}  y {:0.5f}".format(i,i+1, ai_segments[i][-2], ai_segments[i][-1])))
       #for j in range(segment_nb):
       #  print("dbg269: pt {:d}  x {:0.3f}  y {:0.3f}".format(j, ai_segments[j][-2], ai_segments[j][-1]))
       sys.exit(2)
@@ -458,7 +459,7 @@ def outline_circle(ai_center, ai_radius, ai_backend):
   #r_outline = ''
   # check the radius
   if(ai_radius<=0):
-    print("ERR409: Error, the radius {:0.3f} is negative or null!".format(ai_radius))
+    six.print_(("ERR409: Error, the radius {:0.3f} is negative or null!".format(ai_radius)))
     sys.exit(2)
   # select backend
   if(ai_backend=='freecad'):
@@ -506,7 +507,7 @@ def outline_arc_line(ai_segments, ai_backend):
   if(isinstance(ai_segments[0], (tuple, list))): # general outline
     # checks on ai_segments for general outline
     if(len(ai_segments)<2):
-      print("ERR509: Error, the segment list must contain at least 2 elements. Currently, len(ai_segments) = {:d}".format(len(ai_segments)))
+      six.print_(("ERR509: Error, the segment list must contain at least 2 elements. Currently, len(ai_segments) = {:d}".format(len(ai_segments))))
       sys.exit(2)
     # convert any format into format-B
     if(len(ai_segments[0])==3): # format-A or format-C
@@ -515,11 +516,11 @@ def outline_arc_line(ai_segments, ai_backend):
     else:
       outline_B = ai_segments
     if(len(outline_B[0])!=2):
-      print("ERR403: Error, the first element of the segment list must have 2 elements. Currently, len(outline_B[0]) = {:d}".format(len(outline_B[0])))
+      six.print_(("ERR403: Error, the first element of the segment list must have 2 elements. Currently, len(outline_B[0]) = {:d}".format(len(outline_B[0]))))
       sys.exit(2)
     for i in range(len(outline_B)):
       if((len(outline_B[i])!=2)and(len(outline_B[i])!=4)):
-        print("ERR405: Error, the length of the segment {:d} must be 2 or 4. Currently len(outline_B[i]) = {:d}".format(i, len(outline_B[i])))
+        six.print_(("ERR405: Error, the length of the segment {:d} must be 2 or 4. Currently len(outline_B[i]) = {:d}".format(i, len(outline_B[i]))))
         sys.exit(2)
     # check if the outline is closed
     outline_closed = False
@@ -537,8 +538,8 @@ def outline_arc_line(ai_segments, ai_backend):
       r_outline = outline_arc_line_with_tkinter(outline_B, outline_closed)
   else: # circle outline
     if(len(ai_segments)!=3):
-      print("ERR658: Error, circle outline must be a list of 3 floats (or int)! Current len: {:d}".format(len(ai_segments)))
-      print("dbg368: ai_segments:", ai_segments)
+      six.print_(("ERR658: Error, circle outline must be a list of 3 floats (or int)! Current len: {:d}".format(len(ai_segments))))
+      six.print_(("dbg368: ai_segments:", ai_segments))
       sys.exit(2)
     r_outline = outline_circle((ai_segments[0], ai_segments[1]), ai_segments[2], ai_backend)
   return(r_outline)
@@ -559,7 +560,7 @@ def figure_simple_display(ai_figure, ai_overlay_figure=[], ai_parameter_info="")
   i = 0
   for i_outline in ai_figure:
     if(cnc_outline.check_outline_format(i_outline)==2):
-      print("WARN441: Warning, the outline {:d} must be converted in format-B with ideal_outline()!".format(i))
+      six.print_(("WARN441: Warning, the outline {:d} must be converted in format-B with ideal_outline()!".format(i)))
       graphic_figure.append(cnc_outline.ideal_outline(i_outline, "figure_simple_display"))
     else:
       graphic_figure.append(i_outline)
@@ -569,13 +570,13 @@ def figure_simple_display(ai_figure, ai_overlay_figure=[], ai_parameter_info="")
   i = 0
   for i_outline in ai_overlay_figure:
     if(cnc_outline.check_outline_format(i_outline)==2):
-      print("WARN442: Warning, the overlay outline {:d} must be converted in format-B with ideal_outline()!".format(i))
+      six.print_(("WARN442: Warning, the overlay outline {:d} must be converted in format-B with ideal_outline()!".format(i)))
       overlay_figure.append(cnc_outline.ideal_outline(i_outline, "figure_simple_display_overlay"))
     else:
       overlay_figure.append(i_outline)
     i += 1
   # start GUI
-  tk_root = Tkinter.Tk()
+  tk_root = tkinter.Tk()
   fsd_canvas = Two_Canvas(tk_root)
   # callback function for display_backend
   def sub_fsd_canvas_graphics(ai_rotation_direction, ai_angle_position):
@@ -601,7 +602,7 @@ def sub_fsd_canvas_graphics(ai_rotation_direction, ai_angle_position):
 def write_figure_in_svg(ai_figure, ai_filename):
   """ Generate the SVG file ai_filename from the figure ai_figure (list of format B outline)
   """
-  print("Generate with mozman svgwrite the SVG file {:s}".format(ai_filename))
+  six.print_(("Generate with mozman svgwrite the SVG file {:s}".format(ai_filename)))
   object_svg = svgwrite.Drawing(filename = ai_filename)
   for i_ol in ai_figure:
     svg_outline = outline_arc_line(i_ol, 'svgwrite')
@@ -613,7 +614,7 @@ def write_figure_in_svg(ai_figure, ai_filename):
 def write_figure_in_dxf(ai_figure, ai_filename):
   """ Generate the DXF file ai_filename from the figure ai_figure (list of format B outline)
   """
-  print("Generate with mozman dxfwrite the DXF file {:s}".format(ai_filename))
+  six.print_(("Generate with mozman dxfwrite the DXF file {:s}".format(ai_filename)))
   object_dxf = DXFEngine.drawing(ai_filename)
   #object_dxf.add_layer("my_dxf_layer")
   for i_ol in ai_figure:
@@ -646,7 +647,7 @@ def figure_to_freecad_25d_part(ai_figure, ai_extrude_height):
       #print("dbg663: outline with {:d} segments".format(len(ol)-1))
       if((ol[0][0]!=ol[-1][-2])or(ol[0][1]!=ol[-1][-1])):
         face_nwire = False
-        print("WARN504: Warning, the outline {:d} is not closed! Only wire can be extruded.".format(oli+1))
+        six.print_(("WARN504: Warning, the outline {:d} is not closed! Only wire can be extruded.".format(oli+1)))
   # create the FreeCAD part
   if(face_nwire): # generate a real solid part
     outer_face = Part.Face(Part.Wire(outline_arc_line(ai_figure[0], 'freecad').Edges))
@@ -751,7 +752,7 @@ def outline_arc_line_test1():
   Part.show(r_test_solid)
   # create the output directory
   l_output_dir = "test_output"
-  print("Create the output directory: {:s}".format(l_output_dir))
+  six.print_(("Create the output directory: {:s}".format(l_output_dir)))
   design_help.mkdir_p(l_output_dir)
   # backend svgwrite
   print("dbg702: test1 backend svgwrite")
@@ -783,7 +784,7 @@ def outline_arc_line_test1():
   object_dxf.save()
   # backend tkinter
   print("dbg704: test1 backend tkinter")
-  tk_root = Tkinter.Tk()
+  tk_root = tkinter.Tk()
   #my_canvas = display_backend.Two_Canvas(tk_root)
   my_canvas = Two_Canvas(tk_root)
   # callback function for display_backend
@@ -829,16 +830,16 @@ def sub_canvas_graphics(ai_rotation_direction, ai_angle_position):
   wfl_part = figure_to_freecad_25d_part(wfl_figure, wfl_extrude_height)
   
   # output file with mozman
-  print("Generate {:s}/obt1_with_mozman.svg".format(l_output_dir))
+  six.print_(("Generate {:s}/obt1_with_mozman.svg".format(l_output_dir)))
   write_figure_in_svg(wfl_figure, "{:s}/obt1_with_mozman.svg".format(l_output_dir))
-  print("Generate {:s}/obt1_with_mozman.dxf".format(l_output_dir))
+  six.print_(("Generate {:s}/obt1_with_mozman.dxf".format(l_output_dir)))
   write_figure_in_dxf(wfl_figure, "{:s}/obt1_with_mozman.dxf".format(l_output_dir))
 
   # wfl_part in 3D BRep
-  print("Generate {:s}/obt1_part.brep".format(l_output_dir))
+  six.print_(("Generate {:s}/obt1_part.brep".format(l_output_dir)))
   wfl_part.exportBrep("{:s}/obt1_part.brep".format(l_output_dir))
   # wfl_part in 2D DXF
-  print("Generate {:s}/obt1_part.dxf".format(l_output_dir))
+  six.print_(("Generate {:s}/obt1_part.dxf".format(l_output_dir)))
   export_2d.export_to_dxf(wfl_part, Base.Vector(0,0,1), wfl_extrude_height/2, "{:s}/obt1_part.dxf".format(l_output_dir)) # slice wfl_part in the XY plan at a height of wfl_extrude_height/2
   #
   r_test = 1
diff --git a/cnc25d/positioning.py b/cnc25d/positioning.py
index f46d111..a40fea7 100644
--- a/cnc25d/positioning.py
+++ b/cnc25d/positioning.py
@@ -28,7 +28,8 @@
 # header for Python / FreeCAD compatibility
 ################################################################
 
-import importing_freecad
+from . import importing_freecad
+import six
 importing_freecad.importing_freecad()
 
 #print("FreeCAD.Version:", FreeCAD.Version())
@@ -42,7 +43,7 @@
 from FreeCAD import Base
 import math
 import sys, argparse
-import design_help # just for get_effective_args()
+from . import design_help # just for get_effective_args()
 
 ################################################################
 # Positioning API
@@ -66,7 +67,7 @@ def place_plank(ai_plank_solid, ai_x_length, ai_y_width, ai_z_height, ai_flip, a
   elif(ai_flip=='z'):
     r_placed_plank.rotate(Base.Vector(flip_center_x, flip_center_y, flip_center_z),Base.Vector(0,0,1),180)
   else:
-    print("ERR505: Error, the flip value %s doesn't exist! Use only: i,x,y,z."%ai_flip)
+    six.print_(("ERR505: Error, the flip value %s doesn't exist! Use only: i,x,y,z."%ai_flip))
     sys.exit(2)
   # orientation
   if(ai_orientation=='xy'):
@@ -90,7 +91,7 @@ def place_plank(ai_plank_solid, ai_x_length, ai_y_width, ai_z_height, ai_flip, a
     r_placed_plank.rotate(Base.Vector(0, 0, 0),Base.Vector(0,1,0),-90)
     r_placed_plank.translate(Base.Vector(ai_z_height, 0, 0))
   else:
-    print("ERR506: Error, the orientation value %s doesn't exist! Use only: xz,xy,yx,yz,zx,zy."%ai_orientation)
+    six.print_(("ERR506: Error, the orientation value %s doesn't exist! Use only: xz,xy,yx,yz,zx,zy."%ai_orientation))
     sys.exit(2)
   # translation
   r_placed_plank.translate(Base.Vector(ai_translate_x, ai_translate_y, ai_translate_z))
diff --git a/cnc25d/small_geometry.py b/cnc25d/small_geometry.py
index 2816d5c..ebd5040 100644
--- a/cnc25d/small_geometry.py
+++ b/cnc25d/small_geometry.py
@@ -28,7 +28,7 @@
 # python behavior
 ################################################################
 
-from __future__ import division # to get float division
+ # to get float division
 
 
 ################################################################
@@ -39,6 +39,7 @@
 #
 import math
 import sys, argparse
+import six
 
 ################################################################
 # functions to be used by cnc_cut_outline.py
@@ -72,7 +73,7 @@ def arc_center_radius(ai_arc_pt1, ai_arc_pt2, ai_arc_pt3, ai_error_msg_id):
   BC = math.sqrt((CX-BX)**2+(CY-BY)**2)
   AC = math.sqrt((CX-AX)**2+(CY-AY)**2)
   if((AB<radian_epsilon)or(BC<radian_epsilon)or(AC<radian_epsilon)):
-    print("ERR682: Error in {:s}, the three arc points ABC are too closed: AB={:0.2f} BC={:0.2f} AC={:0.2f}".format(ai_error_msg_id, AB, BC, AC))
+    six.print_(("ERR682: Error in {:s}, the three arc points ABC are too closed: AB={:0.2f} BC={:0.2f} AC={:0.2f}".format(ai_error_msg_id, AB, BC, AC)))
     sys.exit(2)
   # calculation of M and N
   MX = (AX+BX)/2
@@ -96,7 +97,7 @@ def arc_center_radius(ai_arc_pt1, ai_arc_pt2, ai_arc_pt3, ai_error_msg_id):
   ixk = sin_f*(cos_e*MX+sin_e*MY)-sin_e*(cos_f*NX+sin_f*NY)
   iyk = cos_f*(cos_e*MX+sin_e*MY)-cos_e*(cos_f*NX+sin_f*NY)
   if((abs(ixl)<radian_epsilon)or(abs(iyl)<radian_epsilon)):
-    print("ERR947: Error in {:s}, ixl (= {:0.2f}) or iyl (= {:0.2f}) are too closed to zero!".format(ai_error_msg_id, ixl, iyl))
+    six.print_(("ERR947: Error in {:s}, ixl (= {:0.2f}) or iyl (= {:0.2f}) are too closed to zero!".format(ai_error_msg_id, ixl, iyl)))
     sys.exit(2)
   IX=ixk/ixl
   IY=iyk/iyl
@@ -105,11 +106,11 @@ def arc_center_radius(ai_arc_pt1, ai_arc_pt2, ai_arc_pt3, ai_error_msg_id):
   IB = math.sqrt((BX-IX)**2+(BY-IY)**2)
   IC = math.sqrt((CX-IX)**2+(CY-IY)**2)
   if((abs(IB-IA)>radian_epsilon)or(abs(IC-IA)>radian_epsilon)):
-    print("ERR748: Error in {:s}, the calculation of the center of the arc A,B,C is wrong! IA={:0.2f} IB={:0.2f} IC={:0.2f}".format(ai_error_msg_id, IA, IB, IC))
-    print("dbg253: A= {:0.2f} {:0.2f}  B= {:0.2f} {:0.2f}  C= {:0.2f} {:0.2f}  I= {:0.2f} {:0.2f}".format(AX,AY,BX,BY,CX,CY,IX,IY))
-    print("dbg764: cos_e={:0.2f}  sin_e={:0.2f}  cos_f={:0.2f}  sin_f={:0.2f}".format(cos_e, sin_e, cos_f, sin_f))
-    print("dbg765: ixl={:0.2f} ixk={:0.2f} iyl={:0.2f} iyk={:0.2f}".format(ixl, ixk, iyl, iyk))
-    print("dbg766: MX={:0.2f} MY={:0.2f} NX={:0.2f} NY={:0.2f}".format(MX,MY,NX,NY))
+    six.print_(("ERR748: Error in {:s}, the calculation of the center of the arc A,B,C is wrong! IA={:0.2f} IB={:0.2f} IC={:0.2f}".format(ai_error_msg_id, IA, IB, IC)))
+    six.print_(("dbg253: A= {:0.2f} {:0.2f}  B= {:0.2f} {:0.2f}  C= {:0.2f} {:0.2f}  I= {:0.2f} {:0.2f}".format(AX,AY,BX,BY,CX,CY,IX,IY)))
+    six.print_(("dbg764: cos_e={:0.2f}  sin_e={:0.2f}  cos_f={:0.2f}  sin_f={:0.2f}".format(cos_e, sin_e, cos_f, sin_f)))
+    six.print_(("dbg765: ixl={:0.2f} ixk={:0.2f} iyl={:0.2f} iyk={:0.2f}".format(ixl, ixk, iyl, iyk)))
+    six.print_(("dbg766: MX={:0.2f} MY={:0.2f} NX={:0.2f} NY={:0.2f}".format(MX,MY,NX,NY)))
     sys.exit(2)
   # return
   r_arc_center_radius=(IX, IY, IA)
@@ -134,7 +135,7 @@ def arc_center_radius_angles(ai_arc_pt1, ai_arc_pt2, ai_arc_pt3, ai_error_msg_id
   IB = math.sqrt((BX-IX)**2+(BY-IY)**2)
   IC = math.sqrt((CX-IX)**2+(CY-IY)**2)
   if((abs(IA-arc_radius)>radian_epsilon)or(abs(IB-arc_radius)>radian_epsilon)or(abs(IC-arc_radius)>radian_epsilon)):
-    print("ERR841: Error, in {:s}, I is not equidistant from A,B,C. arc_radius={:0.2f} IA={:0.2f} IB={:0.2f} IC={:0.2f}".format(ai_error_msg_id, arc_radius, IA, IB, IC))
+    six.print_(("ERR841: Error, in {:s}, I is not equidistant from A,B,C. arc_radius={:0.2f} IA={:0.2f} IB={:0.2f} IC={:0.2f}".format(ai_error_msg_id, arc_radius, IA, IB, IC)))
     sys.exit(2)
   # calculation of the angle u=(Ix, IA) , v=(Ix, IB), w=(Ix, IC), d=(Ix, ID) and e=(Ix, IE)
   u = math.atan2(AY-IY, AX-IX)
@@ -169,7 +170,7 @@ def triangulation(ai_A, ai_AC, ai_B, ai_BC, ai_D, ai_D_direction, ai_error_msg_i
   r_status = 0
   # check the arguments
   if((ai_AC<radian_epsilon)or(ai_BC<radian_epsilon)):
-    print("ERR965: Error in {:s}, the length ai_AC (={:0.2f}) or ai_BC (={:0.2f})".format(ai_error_msg_id, ai_AC, ai_BC))
+    six.print_(("ERR965: Error in {:s}, the length ai_AC (={:0.2f}) or ai_BC (={:0.2f})".format(ai_error_msg_id, ai_AC, ai_BC)))
     sys.exir(2)
   # interprete the arguments
   AX = ai_A[0]
@@ -183,14 +184,14 @@ def triangulation(ai_A, ai_AC, ai_B, ai_BC, ai_D, ai_D_direction, ai_error_msg_i
   # calculation of the length c=AB
   c = math.sqrt((BX-AX)**2+(BY-AY)**2)
   if(c<radian_epsilon):
-    print("ERR662: Error in {:s}, the length c (=AB={:0.2f}) is too small!".format(ai_error_msg_id, c))
+    six.print_(("ERR662: Error in {:s}, the length c (=AB={:0.2f}) is too small!".format(ai_error_msg_id, c)))
     sys.exit(2)
   # calculation of the angle A with the law of cosines
   #BAC = math.acos((b**2+c**2-a**2)/(2*b*c))
   cos_BAC = (b**2+c**2-a**2)/(2*b*c)
   if(abs(cos_BAC)>1):
-    print("ERR542: Error in {:s}, cos_BAC is out of [-1,1]! a={:0.2f} b={:0.2f} c={:0.2f} cos_BAC={:0.2f}".format(ai_error_msg_id, a, b, c, cos_BAC))
-    print("dbg652: AX={:0.2f} AY={:0.2f} BX={:0.2f} BY={:0.2f}".format(AX,AY,BX,BY))
+    six.print_(("ERR542: Error in {:s}, cos_BAC is out of [-1,1]! a={:0.2f} b={:0.2f} c={:0.2f} cos_BAC={:0.2f}".format(ai_error_msg_id, a, b, c, cos_BAC)))
+    six.print_(("dbg652: AX={:0.2f} AY={:0.2f} BX={:0.2f} BY={:0.2f}".format(AX,AY,BX,BY)))
     #sys.exit(2)
     r_status=2
     return((0,0,r_status))
@@ -201,7 +202,7 @@ def triangulation(ai_A, ai_AC, ai_B, ai_BC, ai_D, ai_D_direction, ai_error_msg_i
   xAD =  math.atan2(DY-AY, DX-AX) # atan2 avoid the problem of divided by zero
   BAD = math.fmod(xAD - xAB + 5*math.pi, 2*math.pi) - math.pi
   if(abs(BAD)<radian_epsilon):
-    print("WARN546: Warning in {:s}, the side of the triangulation is not clear! Use the ai_D_direction".format(ai_error_msg_id))
+    six.print_(("WARN546: Warning in {:s}, the side of the triangulation is not clear! Use the ai_D_direction".format(ai_error_msg_id)))
     DX1 = DX+5*radian_epsilon*math.cos(ai_D_direction)
     DY1 = DY+5*radian_epsilon*math.sin(ai_D_direction)
     xAD =  math.atan2(DY1-AY, DX1-AX) # atan2 avoid the problem of divided by zero
@@ -218,7 +219,7 @@ def triangulation(ai_A, ai_AC, ai_B, ai_BC, ai_D, ai_D_direction, ai_error_msg_i
   xBD = math.atan2(DY-BY, DX-BX)
   ABD = math.fmod(xBD - xBA + 5*math.pi, 2*math.pi) - math.pi
   if(abs(ABD)<radian_epsilon):
-    print("WARN547: Warning in {:s}, the side of the triangulation is not clear! Use the ai_D_direction".format(ai_error_msg_id))
+    six.print_(("WARN547: Warning in {:s}, the side of the triangulation is not clear! Use the ai_D_direction".format(ai_error_msg_id)))
     DX1 = DX+5*radian_epsilon*math.cos(ai_D_direction)
     DY1 = DY+5*radian_epsilon*math.sin(ai_D_direction)
     xBD = math.atan2(DY1-BY, DX1-BX)
@@ -227,20 +228,20 @@ def triangulation(ai_A, ai_AC, ai_B, ai_BC, ai_D, ai_D_direction, ai_error_msg_i
   CX2 = BX+a*math.cos(xBC)
   CY2 = BY+a*math.sin(xBC)
   if((abs(CX2-CX)>radian_epsilon)or(abs(CY2-CY)>radian_epsilon)):
-    print("ERR686: Error in {:s}, the coordinate of C seems wrong! CX={:0.2f} CX2={:0.2f} CY={:0.2f} CY2={:0.2f}".format(ai_error_msg_id, CX, CX2, CY, CY2))
-    print("dbg545: D {:0.2f} {:0.2f}  ai_D_direction {:0.2f}".format(DX,DY, ai_D_direction))
-    print("dbg512: BAC", BAC)
-    print("dbg513: xAB", xAB)
+    six.print_(("ERR686: Error in {:s}, the coordinate of C seems wrong! CX={:0.2f} CX2={:0.2f} CY={:0.2f} CY2={:0.2f}".format(ai_error_msg_id, CX, CX2, CY, CY2)))
+    six.print_(("dbg545: D {:0.2f} {:0.2f}  ai_D_direction {:0.2f}".format(DX,DY, ai_D_direction)))
+    six.print_(("dbg512: BAC", BAC))
+    six.print_(("dbg513: xAB", xAB))
     #print("dbg514: AD", AD)
-    print("dbg515: BAD", BAD)
-    print("dbg516: xAC", xAC)
-    print("dbg517: AX {:0.2f}  AY {:0.2f}  b {:0.2f}".format(AX,AY, b))
-    print("dbg522: ABC", ABC)
-    print("dbg523: xBA", xBA)
+    six.print_(("dbg515: BAD", BAD))
+    six.print_(("dbg516: xAC", xAC))
+    six.print_(("dbg517: AX {:0.2f}  AY {:0.2f}  b {:0.2f}".format(AX,AY, b)))
+    six.print_(("dbg522: ABC", ABC))
+    six.print_(("dbg523: xBA", xBA))
     #print("dbg524: BD", BD)
-    print("dbg525: ABD", ABD)
-    print("dbg526: xBC", xBC)
-    print("dbg527: BX {:0.2f}  BY {:0.2f}  a {:0.2f}".format(BX,BY, a))
+    six.print_(("dbg525: ABD", ABD))
+    six.print_(("dbg526: xBC", xBC))
+    six.print_(("dbg527: BX {:0.2f}  BY {:0.2f}  a {:0.2f}".format(BX,BY, a)))
     sys.exit(2)
   r_C = (CX,CY,r_status)
   return(r_C)
@@ -258,7 +259,7 @@ def line_equation(ai_A, ai_B, ai_error_msg_id):
   # calculation of the length AB
   lAB = math.sqrt((BX-AX)**2+(BY-AY)**2)
   if(lAB<radian_epsilon):
-    print("ERR261: Error, lAB {:0.3f} is too small".format(lAB))
+    six.print_(("ERR261: Error, lAB {:0.3f} is too small".format(lAB)))
     sys.exit(2)
   # calculation of the inclination of AB
   xAB = math.atan2(BY-AY, BX-AX)
@@ -351,11 +352,11 @@ def line_circle_intersection(ai_AB, ai_I, ai_R, ai_C, ai_D_direction, ai_error_m
   if(IP>ai_R):
     # This error might occur when the sign of the tangent is wrong because of calculation impression.
     # That's why, we need to report the error instead of stopping on this error
-    print("ERR672: Error in {:s}, the line and the circle have no intersection! IP={:0.2f}  ai_R={:0.2f}".format(ai_error_msg_id, IP, ai_R))
-    print("dbg611: ai_AB:", ai_AB)
-    print("dbg612: ai_I:", ai_I)
-    print("dbg613: ai_R:", ai_R)
-    print("dbg614: ai_C:", ai_C)
+    six.print_(("ERR672: Error in {:s}, the line and the circle have no intersection! IP={:0.2f}  ai_R={:0.2f}".format(ai_error_msg_id, IP, ai_R)))
+    six.print_(("dbg611: ai_AB:", ai_AB))
+    six.print_(("dbg612: ai_I:", ai_I))
+    six.print_(("dbg613: ai_R:", ai_R))
+    six.print_(("dbg614: ai_C:", ai_C))
     #sys.exit(2)
     line_circle_intersection_status=2
     r_line_circle_intersection = (0, 0, line_circle_intersection_status)
@@ -367,7 +368,7 @@ def line_circle_intersection(ai_AB, ai_I, ai_R, ai_C, ai_D_direction, ai_error_m
     cos_i = (C2X-PX)/C2P
     sin_i = (C2Y-PY)/C2P
     if(C2P<radian_epsilon):
-      print("WARN348: Warning in {:s}, C is too closed from the board to find the line_circle_intersection!".format(ai_error_msg_id))
+      six.print_(("WARN348: Warning in {:s}, C is too closed from the board to find the line_circle_intersection!".format(ai_error_msg_id)))
       xAB = math.atan2(a, -b)
       direction_correlation = math.fmod(ai_D_direction-xAB+5*math.pi, 2*math.pi)-math.pi
       direction_AB = math.copysign(1, direction_correlation)
@@ -402,7 +403,7 @@ def line_line_intersection(ai_AB, ai_CD, ai_error_msg_id):
   MY = 0
   determinant = a1*b2-a2*b1
   if(abs(determinant)<radian_epsilon):
-    print("WARN656: Warning in {:s}, the two lines are parallel!".format(ai_error_msg_id))
+    six.print_(("WARN656: Warning in {:s}, the two lines are parallel!".format(ai_error_msg_id)))
     line_line_intersection_status=2
   else:
     MX = (c2*b1-c1*b2)/determinant
@@ -437,7 +438,7 @@ def sub_smooth_corner_line_arc(ai_pre_point, ai_current_point, ai_post_middle, a
   #r_outline = []
   smooth_status = 0
   if(abs(tangent_angle)<radian_epsilon):
-    print("WARN942: Warning in {:s}, the tangent_angle is too flat! the corner doesn't need to be smoothed.".format(ai_error_msg_id))
+    six.print_(("WARN942: Warning in {:s}, the tangent_angle is too flat! the corner doesn't need to be smoothed.".format(ai_error_msg_id)))
     #print("dbg441: ai_pre_point:", ai_pre_point)
     #print("dbg442: ai_current_point:", ai_current_point)
     #print("dbg443: ai_post_middle:", ai_post_middle)
@@ -482,26 +483,26 @@ def sub_smooth_corner_line_arc(ai_pre_point, ai_current_point, ai_post_middle, a
     # when the angle is too sharp, the sign of the tangeant angle might be wrong because of calculation imprecition.
     # in this case, we use the method of try and retry
     if((line_circle_intersection_status==2)and(abs(tangent_angle)>math.pi-10*radian_epsilon)): # error then retry with -Q1_plus instead of Q1_plus
-      print("WARN682: Warning in {:s}, line_arc corner is smoothed with the other side because of a line_circle_intersection error!".format(ai_error_msg_id))
+      six.print_(("WARN682: Warning in {:s}, line_arc corner is smoothed with the other side because of a line_circle_intersection error!".format(ai_error_msg_id)))
       (QX,QY, ACkQ) = line_distance_point(ai_pre_point, ai_current_point, -Q1_plus*ai_router_bit_request, ai_error_msg_id)
       tmpCX = CX + JS*math.cos(xAC+math.pi)
       tmpCY = CY + JS*math.sin(xAC+math.pi)
       (SX,SY, line_circle_intersection_status) = line_circle_intersection((AClx, ACly, ACkQ), (JX,JY),JS, (tmpCX,tmpCY), D_direction, ai_error_msg_id)
     # end of the retry. Continue the normal calculation recipe
     if(line_circle_intersection_status==2):
-      print("WARN684: Warning in {:s}, corner is not smoothed because of a line_circle_intersection error!".format(ai_error_msg_id))
-      print("dbg681: ai_pre_point", ai_pre_point)
-      print("dbg683: ai_current_point", ai_current_point)
-      print("dbg684: ai_post_middle", ai_post_middle)
-      print("dbg685: ai_post_point", ai_post_point)
-      print("dbg686: ai_router_bit_request", ai_router_bit_request)
-      print("dbg623: AClx={:0.2f} ACly={:0.2f} ACkA={:0.2f} lAC={:0.2f} xAC={:0.2f}".format(AClx, ACly, ACkA, lAC, xAC))
-      print("dbg688: JX={:0.2f} JY={:0.2f} R2={:0.2f} uw2={:0.2f} u2={:0.2f}  w2={:0.2f}".format(JX, JY, R2, uw2, u2, w2))
-      print("dbg689: o2={:0.2f} o3={:0.2f}".format(o2,o3))
-      print("dbg691: tangent_angle", tangent_angle)
-      print("dbg692: JS={:0.2f}".format(JS))
-      print("dbg711: QX={:0.2f} QY={:0.2f} ACkQ={:0.2f}".format(QX,QY,ACkQ))
-      print("dbg693: in {:s}, xAC={:0.2f} R2={:0.2f} ai_router_bit_request={:0.2f}".format(ai_error_msg_id, xAC, R2, ai_router_bit_request))
+      six.print_(("WARN684: Warning in {:s}, corner is not smoothed because of a line_circle_intersection error!".format(ai_error_msg_id)))
+      six.print_(("dbg681: ai_pre_point", ai_pre_point))
+      six.print_(("dbg683: ai_current_point", ai_current_point))
+      six.print_(("dbg684: ai_post_middle", ai_post_middle))
+      six.print_(("dbg685: ai_post_point", ai_post_point))
+      six.print_(("dbg686: ai_router_bit_request", ai_router_bit_request))
+      six.print_(("dbg623: AClx={:0.2f} ACly={:0.2f} ACkA={:0.2f} lAC={:0.2f} xAC={:0.2f}".format(AClx, ACly, ACkA, lAC, xAC)))
+      six.print_(("dbg688: JX={:0.2f} JY={:0.2f} R2={:0.2f} uw2={:0.2f} u2={:0.2f}  w2={:0.2f}".format(JX, JY, R2, uw2, u2, w2)))
+      six.print_(("dbg689: o2={:0.2f} o3={:0.2f}".format(o2,o3)))
+      six.print_(("dbg691: tangent_angle", tangent_angle))
+      six.print_(("dbg692: JS={:0.2f}".format(JS)))
+      six.print_(("dbg711: QX={:0.2f} QY={:0.2f} ACkQ={:0.2f}".format(QX,QY,ACkQ)))
+      six.print_(("dbg693: in {:s}, xAC={:0.2f} R2={:0.2f} ai_router_bit_request={:0.2f}".format(ai_error_msg_id, xAC, R2, ai_router_bit_request)))
       #r_outline = [(ai_current_point[0], ai_current_point[1])]
       smooth_status = 2
       sys.exit(2)
@@ -551,11 +552,11 @@ def sub_smooth_corner_arc_arc(ai_pre_point, ai_pre_middle, ai_current_point, ai_
   #r_outline = []
   smooth_status = 0
   if(abs(tangent_angle)<radian_epsilon):
-    print("WARN932: Warning in {:s}, the tangent_angle is too flat! the corner doesn't need to be smoothed.".format(ai_error_msg_id))
+    six.print_(("WARN932: Warning in {:s}, the tangent_angle is too flat! the corner doesn't need to be smoothed.".format(ai_error_msg_id)))
     #r_outline = [(ai_current_point[0], ai_current_point[1])]
     smooth_status = 2
   elif(abs(tangent_angle)>math.pi-radian_epsilon):
-    print("WARN933: Warning in {:s}, the tangent_angle is too sharp! the corner cannot be smoothed.".format(ai_error_msg_id))
+    six.print_(("WARN933: Warning in {:s}, the tangent_angle is too sharp! the corner cannot be smoothed.".format(ai_error_msg_id)))
     #r_outline = [(ai_current_point[0], ai_current_point[1])]
     smooth_status = 2
   else:
@@ -583,7 +584,7 @@ def sub_smooth_corner_arc_arc(ai_pre_point, ai_pre_middle, ai_current_point, ai_
     #print("dbg693: bisector_angle {:0.2f}  D_direction {:0.2f}".format(bisector_angle, D_direction))
     (SX,SY, triangulation_status) = triangulation((IX,IY),IS,(JX,JY),JS,(CX,CY), D_direction, ai_error_msg_id)
     if(triangulation_status==2):
-      print("WARN693: Warning in {:s}, corner is not smoothed because of a triangulation error!".format(ai_error_msg_id))
+      six.print_(("WARN693: Warning in {:s}, corner is not smoothed because of a triangulation error!".format(ai_error_msg_id)))
       #print("dbg681: ai_pre_point", ai_pre_point)
       #print("dbg682: ai_pre_middle", ai_pre_middle)
       #print("dbg683: ai_current_point", ai_current_point)
@@ -632,13 +633,13 @@ def sub_enlarge_corner_line_arc(ai_pre_point, ai_current_point, ai_post_middle,
   MKX=0; MKY=0; GX=0; GY=0; HX=0; HY=0; NLX=0; NLY=0; enlarge_type_request_1=0; enlarge_type_request_2=0
   (SX, SY, UX, UY, xSU, xSJ, router_bit_arc_uw, smooth_status) = sub_smooth_corner_line_arc(ai_pre_point, ai_current_point, ai_post_middle, ai_post_point, ai_router_bit_request, ai_error_msg_id)
   if(smooth_status!=1):
-    print("ERR977: Error in {:s}, the sub smooth corner get an error!".format(ai_error_msg_id))
+    six.print_(("ERR977: Error in {:s}, the sub smooth corner get an error!".format(ai_error_msg_id)))
     sys.exit(2)
     enlarge_status = 2
   else:
     #print("dbg553: router_bit_arc_uw:", router_bit_arc_uw)
     if(abs(router_bit_arc_uw)>math.pi):
-      print("ERR877: Error in {:s}, the sub smooth corner is englobed!".format(ai_error_msg_id))
+      six.print_(("ERR877: Error in {:s}, the sub smooth corner is englobed!".format(ai_error_msg_id)))
       sys.exit(2)
       enlarge_status = 2
     elif(abs(router_bit_arc_uw)<radian_epsilon):
@@ -652,7 +653,7 @@ def sub_enlarge_corner_line_arc(ai_pre_point, ai_current_point, ai_post_middle,
       (SClx, SCly, SCkS, lSC, xSC) = line_equation((SX,SY), (CX,CY), ai_error_msg_id)
       (FX,FY, line_circle_intersection_status) = line_circle_intersection((SClx, SCly, SCkS), (CX,CY),ai_router_bit_request, (SX,SY), xSC+math.pi, ai_error_msg_id)
       if(line_circle_intersection_status==2):
-        print("ERR739: Error in {:s} with the line_circle_intersection_status!".format(ai_error_msg_id))
+        six.print_(("ERR739: Error in {:s} with the line_circle_intersection_status!".format(ai_error_msg_id)))
         sys.exit(2)
       (GX,GY, SCkG) = line_distance_point((FX,FY), (CX,CY), -1*corner_orientation*ai_router_bit_request, ai_error_msg_id)
       (HX,HY, SCkH) = line_distance_point((FX,FY), (CX,CY),  1*corner_orientation*ai_router_bit_request, ai_error_msg_id)
@@ -665,13 +666,13 @@ def sub_enlarge_corner_line_arc(ai_pre_point, ai_current_point, ai_post_middle,
 
       #print("dbg321: MX={:0.2f}  MY={:0.2f}  line_circle_intersection={:0.2f}".format(MX, MY, line_circle_intersection_status))
       if(line_line_intersection_status==2):
-        print("ERR736: Error in {:s} with the line_line_intersection_status!".format(ai_error_msg_id))
+        six.print_(("ERR736: Error in {:s} with the line_line_intersection_status!".format(ai_error_msg_id)))
         sys.exit(2)
         enlarge_status = 2
       (NX,NY, line_circle_intersection_status) = line_circle_intersection((SClx, SCly, SCkH), (JX,JY),R2, ((HX+SX)/2,(HY+SY)/2), xSC+math.pi, ai_error_msg_id)
       #print("dbg322: NX={:0.2f}  NY={:0.2f}  line_circle_intersection={:0.2f}".format(NX, NY, line_circle_intersection_status))
       if(line_circle_intersection_status==2):
-        print("ERR733: Error in {:s} with the line_circle_intersection_status!".format(ai_error_msg_id))
+        six.print_(("ERR733: Error in {:s} with the line_circle_intersection_status!".format(ai_error_msg_id)))
         #print("dbg513: G={:0.2f} {:0.2f}  H={:0.2f} {:0.2f}".format(GX, GY, HX, HY))
         sys.exit(2)
         enlarge_status = 2
@@ -704,7 +705,7 @@ def sub_enlarge_corner_line_arc(ai_pre_point, ai_current_point, ai_post_middle,
         (KX,KY, line_circle_intersection_status) = line_circle_intersection((AClx, ACly, ACkC), (FX,FY),ai_router_bit_request, (GX, GY), xSC+math.pi, ai_error_msg_id)
 
         if(line_circle_intersection==2):
-          print("ERR536: Error in {:s} with the triangulation_status!".format(ai_error_msg_id))
+          six.print_(("ERR536: Error in {:s} with the triangulation_status!".format(ai_error_msg_id)))
           sys.exit(2)
         MKX = KX
         MKY = KY
@@ -712,14 +713,14 @@ def sub_enlarge_corner_line_arc(ai_pre_point, ai_current_point, ai_post_middle,
       if(NH_deep>0):
         (LX,LY, triangulation_status) = triangulation((JX,JY),R2, (FX,FY),ai_router_bit_request, (HX,HY), xSC+math.pi, ai_error_msg_id)
         if(triangulation_status==2):
-          print("ERR533: Error in {:s} with the triangulation_status!".format(ai_error_msg_id))
+          six.print_(("ERR533: Error in {:s} with the triangulation_status!".format(ai_error_msg_id)))
           sys.exit(2)
         NLX = LX
         NLY = LY
         enlarge_type_request_2 = 2
       lMN=math.sqrt((NLX-MKX)**2+(NLY-MKY)**2)
       if(lMN<radian_epsilon):
-        print("WARN667: Warning in {:s}, the angle is too flat to be enlarged!".format(ai_error_msg_id))
+        six.print_(("WARN667: Warning in {:s}, the angle is too flat to be enlarged!".format(ai_error_msg_id)))
         enlarge_type_request_1 = 1
         enlarge_type_request_2 = 1
   # return
@@ -748,16 +749,16 @@ def sub_enlarge_corner_arc_arc(ai_pre_point, ai_pre_middle, ai_current_point, ai
   (SX, SY, xSI, xSJ, router_bit_arc_uw, smooth_status) = sub_smooth_corner_arc_arc(ai_pre_point, ai_pre_middle, ai_current_point, ai_post_middle, ai_post_point, ai_router_bit_request, ai_error_msg_id)
   enlarge_status = 0
   if(smooth_status==2):
-    print("ERR927: Error in {:s}, the sub smooth corner gets an error!".format(ai_error_msg_id))
+    six.print_(("ERR927: Error in {:s}, the sub smooth corner gets an error!".format(ai_error_msg_id)))
     sys.exit(2)
   else:
     #print("dbg553: router_bit_arc_uw:", router_bit_arc_uw)
     if(abs(router_bit_arc_uw)>math.pi):
-      print("ERR887: Error in {:s}, the sub smooth corner is englobed!".format(ai_error_msg_id))
+      six.print_(("ERR887: Error in {:s}, the sub smooth corner is englobed!".format(ai_error_msg_id)))
       sys.exit(2)
       enlarge_status = 2
     elif(abs(router_bit_arc_uw)<radian_epsilon):
-      print("WARN877: Error in {:s}, the corner is too flat to be enlarged with this router_bit radius!".format(ai_error_msg_id))
+      six.print_(("WARN877: Error in {:s}, the corner is too flat to be enlarged with this router_bit radius!".format(ai_error_msg_id)))
       enlarge_status = 2
       enlarge_type_request_1 = 1
       enlarge_type_request_2 = 1
@@ -767,7 +768,7 @@ def sub_enlarge_corner_arc_arc(ai_pre_point, ai_pre_middle, ai_current_point, ai
       (SClx, SCly, SCkS, lSC, xSC) = line_equation((SX,SY), (CX,CY), ai_error_msg_id)
       (FX,FY, line_circle_intersection_status) = line_circle_intersection((SClx, SCly, SCkS), (CX,CY),ai_router_bit_request, (SX,SY), xSC+math.pi, ai_error_msg_id)
       if(line_circle_intersection_status==2):
-        print("ERR639: Error in {:s} with the line_circle_intersection_status!".format(ai_error_msg_id))
+        six.print_(("ERR639: Error in {:s} with the line_circle_intersection_status!".format(ai_error_msg_id)))
         sys.exit(2)
       (GX,GY, SCkG) = line_distance_point((FX,FY), (CX,CY), -1*corner_orientation*ai_router_bit_request, ai_error_msg_id)
       (HX,HY, SCkH) = line_distance_point((FX,FY), (CX,CY),  1*corner_orientation*ai_router_bit_request, ai_error_msg_id)
@@ -776,12 +777,12 @@ def sub_enlarge_corner_arc_arc(ai_pre_point, ai_pre_middle, ai_current_point, ai
       (MX,MY, line_circle_intersection_status) = line_circle_intersection((SClx, SCly, SCkG), (IX,IY),R1, ((GX+SX)/2,(GY+SY)/2), xSC+math.pi, ai_error_msg_id)
       #print("dbg321: MX={:0.2f}  MY={:0.2f}  line_circle_intersection={:0.2f}".format(MX, MY, line_circle_intersection_status))
       if(line_circle_intersection_status==2):
-        print("ERR636: Error in {:s} with the line_circle_intersection_status!".format(ai_error_msg_id))
+        six.print_(("ERR636: Error in {:s} with the line_circle_intersection_status!".format(ai_error_msg_id)))
         sys.exit(2)
       (NX,NY, line_circle_intersection_status) = line_circle_intersection((SClx, SCly, SCkH), (JX,JY),R2, ((HX+SX)/2,(HY+SY)/2), xSC+math.pi, ai_error_msg_id)
       #print("dbg322: NX={:0.2f}  NY={:0.2f}  line_circle_intersection={:0.2f}".format(NX, NY, line_circle_intersection_status))
       if(line_circle_intersection_status==2):
-        print("ERR633: Error in {:s} with the line_circle_intersection_status!".format(ai_error_msg_id))
+        six.print_(("ERR633: Error in {:s} with the line_circle_intersection_status!".format(ai_error_msg_id)))
         #print("dbg513: G={:0.2f} {:0.2f}  H={:0.2f} {:0.2f}".format(GX, GY, HX, HY))
         #sys.exit(2)
         enlarge_status = 2
@@ -812,7 +813,7 @@ def sub_enlarge_corner_arc_arc(ai_pre_point, ai_pre_middle, ai_current_point, ai
       if(MG_deep>0):
         (KX,KY, triangulation_status) = triangulation((IX,IY),R1,(FX,FY),ai_router_bit_request,(GX,GY), xSC+math.pi, ai_error_msg_id)
         if(triangulation_status==2):
-          print("ERR536: Error in {:s} with the triangulation_status!".format(ai_error_msg_id))
+          six.print_(("ERR536: Error in {:s} with the triangulation_status!".format(ai_error_msg_id)))
           sys.exit(2)
         MKX = KX
         MKY = KY
@@ -820,14 +821,14 @@ def sub_enlarge_corner_arc_arc(ai_pre_point, ai_pre_middle, ai_current_point, ai
       if(NH_deep>0):
         (LX,LY, triangulation_status) = triangulation((JX,JY),R2,(FX,FY),ai_router_bit_request,(HX,HY), xSC+math.pi, ai_error_msg_id)
         if(triangulation_status==2):
-          print("ERR533: Error in {:s} with the triangulation_status!".format(ai_error_msg_id))
+          six.print_(("ERR533: Error in {:s} with the triangulation_status!".format(ai_error_msg_id)))
           sys.exit(2)
         NLX = LX
         NLY = LY
         enlarge_type_request_2 = 2
       lMN=math.sqrt((NLX-MKX)**2+(NLY-MKY)**2)
       if(lMN<radian_epsilon):
-        print("WARN667: Warning in {:s}, the angle is too flat to be enlarged!".format(ai_error_msg_id))
+        six.print_(("WARN667: Warning in {:s}, the angle is too flat to be enlarged!".format(ai_error_msg_id)))
         enlarge_type_request_1 = 1
         enlarge_type_request_2 = 1
   # return
@@ -858,7 +859,7 @@ def curve_arc(ai_AX, ai_AY, ai_CX, ai_CY, ai_At, ai_router_bit_request, ai_error
   # O intersection of (OI) and (OA). It's the center of the arc
   (OX, OY, line_line_intersection_status) = line_line_intersection((OIlx, OIly, OIk),(OAlx, OAly, OAk), ai_error_msg_id)
   if(line_line_intersection_status==2):
-    print("ERR374: Error in {:s}, the tangent and AC are collinear!".format(ai_error_msg_id))
+    six.print_(("ERR374: Error in {:s}, the tangent and AC are collinear!".format(ai_error_msg_id)))
     sys.exit(2)
   # verification of the distance OA and OC
   lOA = math.sqrt((ai_AX-OX)**2+(ai_AY-OY)**2)
@@ -868,10 +869,10 @@ def curve_arc(ai_AX, ai_AY, ai_CX, ai_CY, ai_At, ai_router_bit_request, ai_error
   #print("dbg356: IX={:0.2f}  IY={:0.2f}".format(IX, IY))
   #print("dbg357: OX={:0.2f}  OY={:0.2f}".format(OX, OY))
   if(abs(lOC-lOA)>radian_epsilon):
-    print("ERR375: Error in {:s}, O is not equidistant from A and C! lOA={:0.2f} l_OC={:0.2f}".format(ai_error_msg_id, lOA, lOC))
+    six.print_(("ERR375: Error in {:s}, O is not equidistant from A and C! lOA={:0.2f} l_OC={:0.2f}".format(ai_error_msg_id, lOA, lOC)))
     sys.exit(2)
   if(lOA<ai_router_bit_request):
-    print("WARN446: Warning in {:s}, the radius_of_curvature is smaller than the router_bit_request! lOA={:0.2f} rbr={:0.2f}".format(ai_error_msg_id, lOA, ai_router_bit_request))
+    six.print_(("WARN446: Warning in {:s}, the radius_of_curvature is smaller than the router_bit_request! lOA={:0.2f} rbr={:0.2f}".format(ai_error_msg_id, lOA, ai_router_bit_request)))
   # calculation of the angle (Ox, OA)
   xOA = math.atan2((ai_AY-OY)/lOA, (ai_AX-OX)/lOA)
   # calculation of the angle (Ox, OC)
@@ -885,7 +886,7 @@ def curve_arc(ai_AX, ai_AY, ai_CX, ai_CY, ai_At, ai_router_bit_request, ai_error
   else:
     AOC = -1*math.fmod(xOA-xOC+4*math.pi, 2*math.pi)
   if(abs(AOC)<radian_epsilon):
-    print("WARN776: Warning in {:s}, the angle AOC is really small!".format(ai_error_msg_id))
+    six.print_(("WARN776: Warning in {:s}, the angle AOC is really small!".format(ai_error_msg_id)))
   # calculation of B
   xOB = xOA + AOC/2
   BX = OX+lOA*math.cos(xOB)
@@ -914,17 +915,17 @@ def unstable_smooth_outline_b_curve(ai_polyline, ai_initial_tangent, ai_precisio
   if((ai_polyline[0][0]==ai_polyline[-1][-2])and(ai_polyline[0][1]==ai_polyline[-1][-1])):
   #if((abs(ai_polyline[0][0]-ai_polyline[-1][-2])<radian_epsilon)and(abs(ai_polyline[0][1]==ai_polyline[-1][-1])<radian_epsilon)):
     outline_closed = True
-    print("WARN994: Warning in {:s}, the curve to smooth is closed and this will be ignored by the unstable_smooth_curve() function".format(ai_error_msg_id))
+    six.print_(("WARN994: Warning in {:s}, the curve to smooth is closed and this will be ignored by the unstable_smooth_curve() function".format(ai_error_msg_id)))
   # number of corners and segments
   point_nb = len(ai_polyline)
   # check the outline point number
   if(point_nb<2):
-    print("ERR209: Error in {:s}, the number of points must be bigger than 2. Currently: {:d}".format(ai_error_msg_id, point_nb))
+    six.print_(("ERR209: Error in {:s}, the number of points must be bigger than 2. Currently: {:d}".format(ai_error_msg_id, point_nb)))
     sys.exit(2)
   # check if the first point is valid
   first_point_len = len(ai_polyline[0])
   if(first_point_len!=2):
-    print("ERR219: Error in {:s}, the first-point of ai_polyline must have exactly 2 elements. Currently: {:d}".format(ai_error_msg_id, first_point_len))
+    six.print_(("ERR219: Error in {:s}, the first-point of ai_polyline must have exactly 2 elements. Currently: {:d}".format(ai_error_msg_id, first_point_len)))
     sys.exit(2)
   # processing initialization
   ti = ai_initial_tangent
@@ -937,7 +938,7 @@ def unstable_smooth_outline_b_curve(ai_polyline, ai_initial_tangent, ai_precisio
     # check the validity of the new segment
     segment_len = len(ai_polyline[i+1])
     if(segment_len!=2):
-      print("ERR229: Error in {:s}, the ai_polyline segment length must be exactly 2. Currently: {:d}".format(i_error_msg_id, segment_len))
+      six.print_(("ERR229: Error in {:s}, the ai_polyline segment length must be exactly 2. Currently: {:d}".format(i_error_msg_id, segment_len)))
       sys.exit(2)
     # geometrical data
     AX = ai_polyline[i][0]
@@ -949,19 +950,19 @@ def unstable_smooth_outline_b_curve(ai_polyline, ai_initial_tangent, ai_precisio
     iAC = math.atan2((CY-AY)/lAC, (CX-AX)/lAC)
     rti = math.fmod(ti-iAC+5*math.pi, 2*math.pi)-math.pi # angle (AC, tangent) between [-pi,pi]
     if(abs(rti)>math.pi-radian_epsilon):
-      print("ERR239: Error in {:s}, AC and the tangent Ti are collinear and in opposite direction. iAC={:0.2f}  ti={:0.2f}".format(i_error_msg_id, iAC, ti))
+      six.print_(("ERR239: Error in {:s}, AC and the tangent Ti are collinear and in opposite direction. iAC={:0.2f}  ti={:0.2f}".format(i_error_msg_id, iAC, ti)))
       sys.exit(2)
     if(abs(rti)>math.pi/3):
-      print("WARN249: Warning in {:s}, AC and the tangent Ti are doing a large angle. Add itermediate points to remove this warning. iAC={:0.2f}  ti={:0.2f}".format(i_error_msg_id, iAC, ti))
+      six.print_(("WARN249: Warning in {:s}, AC and the tangent Ti are doing a large angle. Add itermediate points to remove this warning. iAC={:0.2f}  ti={:0.2f}".format(i_error_msg_id, iAC, ti)))
     if(abs(rti)<radian_epsilon):
-      print("WARN259: Warning in {:s}, AC and the tangent Ti are almost identical. A line is generated for this segment. iAC={:0.2f}  ti={:0.2f}".format(i_error_msg_id, iAC, ti))
+      six.print_(("WARN259: Warning in {:s}, AC and the tangent Ti are almost identical. A line is generated for this segment. iAC={:0.2f}  ti={:0.2f}".format(i_error_msg_id, iAC, ti)))
       r_outline.append([CX, CY]) # create a line-segment
       ti = iAC
     else:
       (BX, BY, nti) = curve_arc(AX, AY, CX, CY, ti, ai_router_bit_request, i_error_msg_id)
       r_outline.append([BX, BY, CX, CY]) # create an arc-segment
       ti = nti
-  print("dbg536: the last tangent inclination ti: {:0.2f}".format(ti))
+  six.print_(("dbg536: the last tangent inclination ti: {:0.2f}".format(ti)))
   # return
   return(r_outline)
 
@@ -987,12 +988,12 @@ def sub_smooth_outline_c_curve(ai_polyline, ai_precision, ai_router_bit_request,
   point_nb = len(ai_polyline)
   # check the outline point number
   if(point_nb<2):
-    print("ERR609: Error in {:s}, the number of points must be bigger than 2. Currently: {:d}".format(ai_error_msg_id, point_nb))
+    six.print_(("ERR609: Error in {:s}, the number of points must be bigger than 2. Currently: {:d}".format(ai_error_msg_id, point_nb)))
     sys.exit(2)
   # check if the first point is valid
   first_point_len = len(ai_polyline[0])
   if(first_point_len!=3):
-    print("ERR619: Error in {:s}, the first-point of ai_polyline must have exactly 3 elements. Currently: {:d}".format(ai_error_msg_id, first_point_len))
+    six.print_(("ERR619: Error in {:s}, the first-point of ai_polyline must have exactly 3 elements. Currently: {:d}".format(ai_error_msg_id, first_point_len)))
     sys.exit(2)
   # processing initialization
   r_outline = []
@@ -1004,7 +1005,7 @@ def sub_smooth_outline_c_curve(ai_polyline, ai_precision, ai_router_bit_request,
     # check the validity of the new segment
     segment_len = len(ai_polyline[i+1])
     if(segment_len!=3):
-      print("ERR629: Error in {:s}, the ai_polyline segment length must be exactly 3. Currently: {:d}".format(i_error_msg_id, segment_len))
+      six.print_(("ERR629: Error in {:s}, the ai_polyline segment length must be exactly 3. Currently: {:d}".format(i_error_msg_id, segment_len)))
       sys.exit(2)
     # geometrical data
     AX = ai_polyline[i][0]
@@ -1030,34 +1031,34 @@ def sub_smooth_outline_c_curve(ai_polyline, ai_precision, ai_router_bit_request,
     ECk = -1*(EClx*EX+ECly*EY)
     # check if the segment must be an arc or a line
     if(abs(AtAE)>math.pi/2):
-      print("ERR639: Error in {:s}, the angle between AC and the tangent xAt is larger than pi/2. It doesn't look like a feasible curbe. xAC={:0.2f}  xAt={:0.2f}".format(i_error_msg_id, xAC, xAt))
+      six.print_(("ERR639: Error in {:s}, the angle between AC and the tangent xAt is larger than pi/2. It doesn't look like a feasible curbe. xAC={:0.2f}  xAt={:0.2f}".format(i_error_msg_id, xAC, xAt)))
       sys.exit(2)
     if(abs(EtEA)>math.pi/2):
-      print("ERR638: Error in {:s}, the angle between EC and the tangent et is larger than pi/2. It doesn't look like a feasible curbe. xEC={:0.2f}  xEt={:0.2f}".format(i_error_msg_id, xEC, xEt))
+      six.print_(("ERR638: Error in {:s}, the angle between EC and the tangent et is larger than pi/2. It doesn't look like a feasible curbe. xEC={:0.2f}  xEt={:0.2f}".format(i_error_msg_id, xEC, xEt)))
       sys.exit(2)
     if(abs(AtAE)>math.pi/3):
-      print("WARN649: Warning in {:s}, AC and the tangent xAt are doing a large angle. Add itermediate points to remove this warning. xAC={:0.2f}  xAt={:0.2f}".format(i_error_msg_id, xAC, xAt))
+      six.print_(("WARN649: Warning in {:s}, AC and the tangent xAt are doing a large angle. Add itermediate points to remove this warning. xAC={:0.2f}  xAt={:0.2f}".format(i_error_msg_id, xAC, xAt)))
     if(abs(EtEA)>math.pi/3):
-      print("WARN648: Warning in {:s}, EC and the tangent xEt are doing a large angle. Add itermediate points to remove this warning. xEC={:0.2f}  xEt={:0.2f}".format(i_error_msg_id, xEC, xEt))
+      six.print_(("WARN648: Warning in {:s}, EC and the tangent xEt are doing a large angle. Add itermediate points to remove this warning. xEC={:0.2f}  xEt={:0.2f}".format(i_error_msg_id, xEC, xEt)))
     if((abs(AtAE)<radian_epsilon)or(abs(EtEA)<radian_epsilon)):
-      print("WARN659: Warning in {:s}, (xAC, xAt) or (xEC, xEt) are almost identical. A line is generated for this segment. xAC={:0.2f}  xAt={:0.2f} xEC={:0.2f}  xEt={:0.2f} ".format(i_error_msg_id, xAC, xAt, xEC, xEt))
+      six.print_(("WARN659: Warning in {:s}, (xAC, xAt) or (xEC, xEt) are almost identical. A line is generated for this segment. xAC={:0.2f}  xAt={:0.2f} xEC={:0.2f}  xEt={:0.2f} ".format(i_error_msg_id, xAC, xAt, xEC, xEt)))
       r_outline.append((EX, EY)) # create a line-segment
     elif((AtAE*EtEA)>0):
-      print("WARN669: Warning in {:s}, xAt and xEt are not one the side of (AE). It look like an inflexion. A line is generated for this segment. xAC={:0.2f}  xAt={:0.2f} xEC={:0.2f} xEt={:0.2f} ".format(i_error_msg_id, xAC, xAt, xEC, xEt))
+      six.print_(("WARN669: Warning in {:s}, xAt and xEt are not one the side of (AE). It look like an inflexion. A line is generated for this segment. xAC={:0.2f}  xAt={:0.2f} xEC={:0.2f} xEt={:0.2f} ".format(i_error_msg_id, xAC, xAt, xEC, xEt)))
       r_outline.append((EX, EY)) # create a line-segment
     else:
       # C intersection of (AC) and (EC). it is the junction point between the two arcs
       (CX, CY, line_line_intersection_status) = line_line_intersection((AClx, ACly, ACk),(EClx, ECly, ECk), ai_error_msg_id)
       if(line_line_intersection_status==2):
-        print("ERR324: Error in {:s}, AC and EC are collinear!".format(ai_error_msg_id))
+        six.print_(("ERR324: Error in {:s}, AC and EC are collinear!".format(ai_error_msg_id)))
         sys.exit(2)
       (BX, BY, xCt) = curve_arc(AX, AY, CX, CY, xAt, ai_router_bit_request, i_error_msg_id)
       if(abs(math.fmod(xCt-xAE+5*math.pi, 2*math.pi)-math.pi)>radian_epsilon):
-        print("ERR325: Error in {:s}, the first arc tangent in C is not parallel to AE! xCt={:0.2f} xAE={:0.2f}".format(ai_error_msg_id, xCt, xAE))
+        six.print_(("ERR325: Error in {:s}, the first arc tangent in C is not parallel to AE! xCt={:0.2f} xAE={:0.2f}".format(ai_error_msg_id, xCt, xAE)))
         sys.exit(2)
       (DX, DY, xEt2) = curve_arc(CX, CY, EX, EY, xAE, ai_router_bit_request, i_error_msg_id)
       if(abs(math.fmod(xEt2-xEt+5*math.pi, 2*math.pi)-math.pi)>radian_epsilon):
-        print("ERR326: Error in {:s}, the second arc tangent in E is different from xEt! xEt={:0.2f} xEt2={:0.2f}".format(ai_error_msg_id, xEt, xEt2))
+        six.print_(("ERR326: Error in {:s}, the second arc tangent in E is different from xEt! xEt={:0.2f} xEt2={:0.2f}".format(ai_error_msg_id, xEt, xEt2)))
         sys.exit(2)
       r_outline.append((BX, BY, CX, CY)) # create the first arc-segment
       r_outline.append((DX, DY, EX, EY)) # create the second arc-segment
diff --git a/cnc25d/split_gearwheel.py b/cnc25d/split_gearwheel.py
index fcfa732..a89a50e 100644
--- a/cnc25d/split_gearwheel.py
+++ b/cnc25d/split_gearwheel.py
@@ -30,7 +30,8 @@
 # header for Python / FreeCAD compatibility
 ################################################################
 
-import cnc25d_api
+from . import cnc25d_api
+import six
 cnc25d_api.importing_freecad()
 
 #print("FreeCAD.Version:", FreeCAD.Version())
@@ -53,7 +54,7 @@
 #import svgwrite
 #from dxfwrite import DXFEngine
 # cnc25d
-import gear_profile
+from . import gear_profile
 
 
 ################################################################
@@ -145,14 +146,14 @@ def split_gearwheel_constraint_check(c):
     c['split_rbr'] = c['cnc_router_bit_radius']
   # c['low_split_type']
   if(not c['low_split_type'] in ('circle', 'line')):
-    print("ERR216: Error, c['low_split_type'] {:s} is not valid!".format(c['low_split_type']))
+    six.print_(("ERR216: Error, c['low_split_type'] {:s} is not valid!".format(c['low_split_type'])))
     sys.exit(2)
   # c['high_split_type']
   if(not c['high_split_type'] in ('h', 'a')):
-    print("ERR220: Error, c['high_split_type'] {:s} is not valid!".format(c['high_split_type']))
+    six.print_(("ERR220: Error, c['high_split_type'] {:s} is not valid!".format(c['high_split_type'])))
     sys.exit(2)
   if(c['split_nb']<2):
-    print("ERR188: Error, split_nb {:d} must be equal or bigger than 2".format(c['split_nb']))
+    six.print_(("ERR188: Error, split_nb {:d} must be equal or bigger than 2".format(c['split_nb'])))
     sys.exit(2)
   #c['portion_angle'] = 2*math.pi/c['split_nb']
   c['portion_angle'] = math.pi/c['split_nb']
@@ -178,7 +179,7 @@ def split_gearwheel_constraint_check(c):
     g1_ia = gear_profile_parameters['g1_param']['initial_angle']
     g1_tl = gear_profile_parameters['g1_param']['top_land']
     if(c['portion_angle']<(1.1*g1_pma)):
-      print("ERR219: Error, portion_angle {:0.3f} is too small compare to the pi_module_angle {:0.3f}".format(c['portion_angle'], g1_pma))
+      six.print_(("ERR219: Error, portion_angle {:0.3f} is too small compare to the pi_module_angle {:0.3f}".format(c['portion_angle'], g1_pma)))
       sys.exit(2)
     # pre common calculation
     absolute_angle = []
@@ -235,7 +236,7 @@ def split_gearwheel_constraint_check(c):
     #print("dbg276: len(portion_gear_first_end) {:d}  len(portion_gear_last_end) {:d}".format(len(c['portion_gear_first_end']), len(c['portion_gear_last_end'])))
   else: # no gear_profile, just a circle
     if(c['gear_primitive_diameter']<radian_epsilon):
-      print("ERR885: Error, the no-gear-profile circle outline diameter c['gear_primitive_diameter'] {:0.2f} is too small!".format(c['gear_primitive_diameter']))
+      six.print_(("ERR885: Error, the no-gear-profile circle outline diameter c['gear_primitive_diameter'] {:0.2f} is too small!".format(c['gear_primitive_diameter'])))
       sys.exit(2)
     #gear_profile_B = (c['g1_ix'], c['g1_iy'], float(c['gear_primitive_diameter'])/2)
     c['minimal_gear_profile_radius'] = float(c['gear_primitive_diameter'])/2
@@ -243,7 +244,7 @@ def split_gearwheel_constraint_check(c):
     c['g1_iy'] = c['center_position_y']
     c['addendum_radius'] = float(c['gear_primitive_diameter'])/2 # for slice_xyz
     if(c['high_split_diameter']!=0):
-      print("WARN221: Warning, the setting high_split_diameter {:0.3f} should not be used when gear_tooth_nb=0".format(c['high_split_diameter']))
+      six.print_(("WARN221: Warning, the setting high_split_diameter {:0.3f} should not be used when gear_tooth_nb=0".format(c['high_split_diameter'])))
     c['high_split_radius'] = c['minimal_gear_profile_radius']
   # set default value (if set to zero) for high_split_diameter, low_hole_circle_diameter, high_hole_circle_diameter
   c['low_split_radius'] = c['low_split_diameter']/2.0
@@ -266,16 +267,16 @@ def split_gearwheel_constraint_check(c):
     c['high_hole_nb']=1
   # radial parameters
   if(c['low_hole_circle_radius']<(c['low_split_radius'] + c['low_hole_radius'])):
-    print("ERR230: Error, low_hole_circle_radius {:0.3f} is too small compare to low_split_radius {:0.3f} and low_hole_radius {:0.3f}".format(c['low_hole_circle_radius'], c['low_split_radius'], c['low_hole_radius']))
+    six.print_(("ERR230: Error, low_hole_circle_radius {:0.3f} is too small compare to low_split_radius {:0.3f} and low_hole_radius {:0.3f}".format(c['low_hole_circle_radius'], c['low_split_radius'], c['low_hole_radius'])))
     sys.exit(2)
   if(c['high_hole_circle_radius']<(c['low_hole_circle_radius'] + c['low_hole_radius'] + c['high_hole_radius'])):
-    print("ERR232: Error, high_hole_circle_radius {:0.3f} is too small compare to low_hole_circle_radius {:0.3f}, low_hole_radius {:0.3f} and high_hole_radius {:0.3f}".format(c['high_hole_circle_radius'], c['low_hole_circle_radius'], c['low_hole_radius'], c['high_hole_radius']))
+    six.print_(("ERR232: Error, high_hole_circle_radius {:0.3f} is too small compare to low_hole_circle_radius {:0.3f}, low_hole_radius {:0.3f} and high_hole_radius {:0.3f}".format(c['high_hole_circle_radius'], c['low_hole_circle_radius'], c['low_hole_radius'], c['high_hole_radius'])))
     sys.exit(2)
   if(c['high_split_radius']<(c['high_hole_circle_radius'] + c['high_hole_radius'])):
-    print("ERR236: Error, high_split_radius {:0.3f} is too small compare to high_hole_circle_radius {:0.3f} and high_hole_radius {:0.3f}".format(c['high_split_radius'], c['high_hole_circle_radius'], c['high_hole_radius']))
+    six.print_(("ERR236: Error, high_split_radius {:0.3f} is too small compare to high_hole_circle_radius {:0.3f} and high_hole_radius {:0.3f}".format(c['high_split_radius'], c['high_hole_circle_radius'], c['high_hole_radius'])))
     sys.exit(2)
   if(c['minimal_gear_profile_radius']<c['high_split_radius']):
-    print("ERR239: Error, minimal_gear_profile_radius {:0.3f} is smaller than high_split_radius {:0.3f}".format(c['minimal_gear_profile_radius'], c['high_split_radius']))
+    six.print_(("ERR239: Error, minimal_gear_profile_radius {:0.3f} is smaller than high_split_radius {:0.3f}".format(c['minimal_gear_profile_radius'], c['high_split_radius'])))
     sys.exit(2)
   # angular (or circumference) parameters
   low_hole_diameter_angle = math.asin(float(c['low_hole_radius'])/c['low_hole_circle_radius'])
@@ -283,10 +284,10 @@ def split_gearwheel_constraint_check(c):
   high_hole_diameter_angle = math.asin(float(c['high_hole_radius'])/c['high_hole_circle_radius'])
   c['high_hole_space_angle'] = c['portion_angle']/float(c['high_hole_nb'])
   if(c['low_hole_space_angle']<(2*low_hole_diameter_angle+radian_epsilon)):
-    print("ERR253: Error, low_hole_nb {:d} or low_hole_diameter {:0.3f} are too big!".format(c['low_hole_nb'], c['low_hole_diameter']))
+    six.print_(("ERR253: Error, low_hole_nb {:d} or low_hole_diameter {:0.3f} are too big!".format(c['low_hole_nb'], c['low_hole_diameter'])))
     sys.exit(2)
   if(c['high_hole_space_angle']<(2*high_hole_diameter_angle+radian_epsilon)):
-    print("ERR255: Error, high_hole_nb {:d} or high_hole_diameter {:0.3f} are too big!".format(c['high_hole_nb'], c['high_hole_diameter']))
+    six.print_(("ERR255: Error, high_hole_nb {:d} or high_hole_diameter {:0.3f} are too big!".format(c['high_hole_nb'], c['high_hole_diameter'])))
     sys.exit(2)
   ###
   return(c)
@@ -308,7 +309,7 @@ def split_gearwheel_2d_construction(c):
       #print("dbg335:  portion_gear_last_end[i]: {:0.3f}".format(c['portion_gear_last_end'][i]))
       #print("dbg336:  portion_gear_tooth_angle[i]: {:0.3f}".format(c['portion_gear_tooth_angle'][i]))
       if(c['portion_gear_tooth_nb'][i]<1):
-        print("ERR338: Error, for i {:d} portion_gear_tooth_nb {:d} smaller than 1".format(i, c['portion_gear_tooth_nb'][i]))
+        six.print_(("ERR338: Error, for i {:d} portion_gear_tooth_nb {:d} smaller than 1".format(i, c['portion_gear_tooth_nb'][i])))
         sys.exit(2)
       gp_c = c.copy()
       gp_c['gear_type'] = 'e'
diff --git a/cnc25d/tests/axle_lid_macro.py b/cnc25d/tests/axle_lid_macro.py
index bdfdce4..9a4375d 100644
--- a/cnc25d/tests/axle_lid_macro.py
+++ b/cnc25d/tests/axle_lid_macro.py
@@ -63,7 +63,7 @@
 try: # when working with an installed Cnc25D package
   from cnc25d import cnc25d_api
 except:    # when working on the source files
-  import importing_cnc25d # give access to the cnc25d package
+  from . import importing_cnc25d # give access to the cnc25d package
   from cnc25d import cnc25d_api
 cnc25d_api.importing_freecad()
 
diff --git a/cnc25d/tests/bagel_macro.py b/cnc25d/tests/bagel_macro.py
index a12be1e..c590d25 100644
--- a/cnc25d/tests/bagel_macro.py
+++ b/cnc25d/tests/bagel_macro.py
@@ -63,7 +63,7 @@
 try: # when working with an installed Cnc25D package
   from cnc25d import cnc25d_api
 except:    # when working on the source files
-  import importing_cnc25d # give access to the cnc25d package
+  from . import importing_cnc25d # give access to the cnc25d package
   from cnc25d import cnc25d_api
 cnc25d_api.importing_freecad()
 
diff --git a/cnc25d/tests/bell_bagel_assembly_macro.py b/cnc25d/tests/bell_bagel_assembly_macro.py
index 7efd11a..a907a15 100644
--- a/cnc25d/tests/bell_bagel_assembly_macro.py
+++ b/cnc25d/tests/bell_bagel_assembly_macro.py
@@ -63,7 +63,7 @@
 try: # when working with an installed Cnc25D package
   from cnc25d import cnc25d_api
 except:    # when working on the source files
-  import importing_cnc25d # give access to the cnc25d package
+  from . import importing_cnc25d # give access to the cnc25d package
   from cnc25d import cnc25d_api
 cnc25d_api.importing_freecad()
 
diff --git a/cnc25d/tests/bell_macro.py b/cnc25d/tests/bell_macro.py
index dfa1e04..9c008cb 100644
--- a/cnc25d/tests/bell_macro.py
+++ b/cnc25d/tests/bell_macro.py
@@ -63,7 +63,7 @@
 try: # when working with an installed Cnc25D package
   from cnc25d import cnc25d_api
 except:    # when working on the source files
-  import importing_cnc25d # give access to the cnc25d package
+  from . import importing_cnc25d # give access to the cnc25d package
   from cnc25d import cnc25d_api
 cnc25d_api.importing_freecad()
 
diff --git a/cnc25d/tests/box_wood_frame_macro.py b/cnc25d/tests/box_wood_frame_macro.py
index 4cb421f..bfa298c 100644
--- a/cnc25d/tests/box_wood_frame_macro.py
+++ b/cnc25d/tests/box_wood_frame_macro.py
@@ -63,7 +63,7 @@
 try: # when working with an installed Cnc25D package
   from cnc25d import cnc25d_api
 except:    # when working on the source files
-  import importing_cnc25d # give access to the cnc25d package
+  from . import importing_cnc25d # give access to the cnc25d package
   from cnc25d import cnc25d_api
 cnc25d_api.importing_freecad()
 
diff --git a/cnc25d/tests/bug_dxf_arc.py b/cnc25d/tests/bug_dxf_arc.py
index 121325c..eb30cdb 100644
--- a/cnc25d/tests/bug_dxf_arc.py
+++ b/cnc25d/tests/bug_dxf_arc.py
@@ -33,7 +33,7 @@
 try: # when working with an installed Cnc25D package
   from cnc25d import cnc25d_api
 except:    # when working on the source files
-  import importing_cnc25d # give access to the cnc25d package
+  from . import importing_cnc25d # give access to the cnc25d package
   from cnc25d import cnc25d_api
 #cnc25d_api.importing_freecad()
 #print("FreeCAD.Version:", FreeCAD.Version())
diff --git a/cnc25d/tests/cnc25d_api_macro.py b/cnc25d/tests/cnc25d_api_macro.py
index 98f49b8..59b92c6 100644
--- a/cnc25d/tests/cnc25d_api_macro.py
+++ b/cnc25d/tests/cnc25d_api_macro.py
@@ -71,7 +71,7 @@
 try: # when working with an installed Cnc25D package
   from cnc25d import cnc25d_api
 except:    # when working on the source files
-  import importing_cnc25d # give access to the cnc25d package
+  from . import importing_cnc25d # give access to the cnc25d package
   from cnc25d import cnc25d_api # import the Cnc25D API modules
 # add the FreeCAD library path to the search path
 cnc25d_api.importing_freecad()
@@ -81,7 +81,8 @@
 #
 #import os, errno # to create the output directory
 import math # to get the pi number
-import Tkinter # to display the outline in a small GUI
+from six.moves import tkinter # to display the outline in a small GUI
+import six
 #import svgwrite
 #from dxfwrite import DXFEngine
 
@@ -181,7 +182,7 @@
 
 ## display my_outline_for_cnc with Tkinter
 print("Display the outlines with Tkinter")
-tk_root = Tkinter.Tk()
+tk_root = tkinter.Tk()
 my_canvas = cnc25d_api.Two_Canvas(tk_root)
 # callback function for display_backend
 def sub_canvas_graphics(ai_rotation_direction, ai_angle_position):
@@ -206,7 +207,7 @@ def sub_canvas_graphics(ai_rotation_direction, ai_angle_position):
 
 #
 l_output_dir = "test_output"
-print("Create the output directory: {:s}".format(l_output_dir))
+six.print_(("Create the output directory: {:s}".format(l_output_dir)))
 cnc25d_api.mkdir_p(l_output_dir)
 
 ################################################################
@@ -277,27 +278,27 @@ def sub_canvas_graphics(ai_rotation_direction, ai_angle_position):
 ################################################################
 
 # my_part in 3D
-print("Generate {:s}/my_part.stl".format(l_output_dir))
+six.print_(("Generate {:s}/my_part.stl".format(l_output_dir)))
 my_part_solid.exportStl("{:s}/my_part.stl".format(l_output_dir))
-print("Generate {:s}/my_part.brep".format(l_output_dir))
+six.print_(("Generate {:s}/my_part.brep".format(l_output_dir)))
 my_part_solid.exportBrep("{:s}/my_part.brep".format(l_output_dir))
 #print("Generate {:s}/my_part.step".format(l_output_dir))
 #my_part_solid.exportStep("{:s}/my_part.step".format(l_output_dir))
 
 # my_part in 2D DXF
-print("Generate {:s}/my_part.dxf".format(l_output_dir))
+six.print_(("Generate {:s}/my_part.dxf".format(l_output_dir)))
 cnc25d_api.export_to_dxf(my_part_solid, Base.Vector(0,0,1), 1.0, "{:s}/my_part.dxf".format(l_output_dir)) # slice my_part in the XY plan at a height of 1.0
 
 # my_assembly in 3D
-print("Generate {:s}/my_assembly.stl".format(l_output_dir))
+six.print_(("Generate {:s}/my_assembly.stl".format(l_output_dir)))
 my_assembly.exportStl("{:s}/my_assembly.stl".format(l_output_dir))
-print("Generate {:s}/my_assembly.brep".format(l_output_dir))
+six.print_(("Generate {:s}/my_assembly.brep".format(l_output_dir)))
 my_assembly.exportBrep("{:s}/my_assembly.brep".format(l_output_dir))
 #print("Generate {:s}/my_assembly.step".format(l_output_dir))
 #my_assembly.exportStep("{:s}/my_assembly.step".format(l_output_dir))
 
 # my_assembly sliced and projected in 2D DXF
-print("Generate {:s}/my_assembly.dxf".format(l_output_dir))
+six.print_(("Generate {:s}/my_assembly.dxf".format(l_output_dir)))
 xy_slice_list = [ 0.1+20*i for i in range(12) ]
 xz_slice_list = [ 0.1+20*i for i in range(9) ]
 yz_slice_list = [ 0.1+20*i for i in range(9) ]
@@ -343,10 +344,10 @@ def sub_canvas_graphics(ai_rotation_direction, ai_angle_position):
 wfl_part = cnc25d_api.figure_to_freecad_25d_part(wfl_figure, wfl_extrude_height)
 
 # wfl_part in 3D BRep
-print("Generate {:s}/wfl_part.brep".format(l_output_dir))
+six.print_(("Generate {:s}/wfl_part.brep".format(l_output_dir)))
 wfl_part.exportBrep("{:s}/wfl_part.brep".format(l_output_dir))
 # wfl_part in 2D DXF
-print("Generate {:s}/wfl_part.dxf".format(l_output_dir))
+six.print_(("Generate {:s}/wfl_part.dxf".format(l_output_dir)))
 cnc25d_api.export_to_dxf(wfl_part, Base.Vector(0,0,1), wfl_extrude_height/2, "{:s}/wfl_part.dxf".format(l_output_dir)) # slice wfl_part in the XY plan at a height of wfl_extrude_height/2
 
 
diff --git a/cnc25d/tests/crest_macro.py b/cnc25d/tests/crest_macro.py
index c0c744c..062484b 100644
--- a/cnc25d/tests/crest_macro.py
+++ b/cnc25d/tests/crest_macro.py
@@ -63,7 +63,7 @@
 try: # when working with an installed Cnc25D package
   from cnc25d import cnc25d_api
 except:    # when working on the source files
-  import importing_cnc25d # give access to the cnc25d package
+  from . import importing_cnc25d # give access to the cnc25d package
   from cnc25d import cnc25d_api
 cnc25d_api.importing_freecad()
 
diff --git a/cnc25d/tests/cross_cube_macro.py b/cnc25d/tests/cross_cube_macro.py
index 8213888..80ec56e 100644
--- a/cnc25d/tests/cross_cube_macro.py
+++ b/cnc25d/tests/cross_cube_macro.py
@@ -63,7 +63,7 @@
 try: # when working with an installed Cnc25D package
   from cnc25d import cnc25d_api
 except:    # when working on the source files
-  import importing_cnc25d # give access to the cnc25d package
+  from . import importing_cnc25d # give access to the cnc25d package
   from cnc25d import cnc25d_api
 cnc25d_api.importing_freecad()
 
diff --git a/cnc25d/tests/epicyclic_gearing_macro.py b/cnc25d/tests/epicyclic_gearing_macro.py
index 19fb8c0..ae8b845 100644
--- a/cnc25d/tests/epicyclic_gearing_macro.py
+++ b/cnc25d/tests/epicyclic_gearing_macro.py
@@ -63,7 +63,7 @@
 try: # when working with an installed Cnc25D package
   from cnc25d import cnc25d_api
 except:    # when working on the source files
-  import importing_cnc25d # give access to the cnc25d package
+  from . import importing_cnc25d # give access to the cnc25d package
   from cnc25d import cnc25d_api
 cnc25d_api.importing_freecad()
 
diff --git a/cnc25d/tests/gear_profile_macro.py b/cnc25d/tests/gear_profile_macro.py
index c96ca0d..7f8459d 100644
--- a/cnc25d/tests/gear_profile_macro.py
+++ b/cnc25d/tests/gear_profile_macro.py
@@ -63,7 +63,7 @@
 try: # when working with an installed Cnc25D package
   from cnc25d import cnc25d_api
 except:    # when working on the source files
-  import importing_cnc25d # give access to the cnc25d package
+  from . import importing_cnc25d # give access to the cnc25d package
   from cnc25d import cnc25d_api
 cnc25d_api.importing_freecad()
 
diff --git a/cnc25d/tests/gearbar_macro.py b/cnc25d/tests/gearbar_macro.py
index 9f1750b..6257092 100644
--- a/cnc25d/tests/gearbar_macro.py
+++ b/cnc25d/tests/gearbar_macro.py
@@ -63,7 +63,7 @@
 try: # when working with an installed Cnc25D package
   from cnc25d import cnc25d_api
 except:    # when working on the source files
-  import importing_cnc25d # give access to the cnc25d package
+  from . import importing_cnc25d # give access to the cnc25d package
   from cnc25d import cnc25d_api
 cnc25d_api.importing_freecad()
 
diff --git a/cnc25d/tests/gearring_macro.py b/cnc25d/tests/gearring_macro.py
index 4e3e0ba..28d0b3b 100644
--- a/cnc25d/tests/gearring_macro.py
+++ b/cnc25d/tests/gearring_macro.py
@@ -63,7 +63,7 @@
 try: # when working with an installed Cnc25D package
   from cnc25d import cnc25d_api
 except:    # when working on the source files
-  import importing_cnc25d # give access to the cnc25d package
+  from . import importing_cnc25d # give access to the cnc25d package
   from cnc25d import cnc25d_api
 cnc25d_api.importing_freecad()
 
diff --git a/cnc25d/tests/gearwheel_macro.py b/cnc25d/tests/gearwheel_macro.py
index 90b2225..06c3876 100644
--- a/cnc25d/tests/gearwheel_macro.py
+++ b/cnc25d/tests/gearwheel_macro.py
@@ -63,7 +63,7 @@
 try: # when working with an installed Cnc25D package
   from cnc25d import cnc25d_api
 except:    # when working on the source files
-  import importing_cnc25d # give access to the cnc25d package
+  from . import importing_cnc25d # give access to the cnc25d package
   from cnc25d import cnc25d_api
 cnc25d_api.importing_freecad()
 
diff --git a/cnc25d/tests/gimbal_macro.py b/cnc25d/tests/gimbal_macro.py
index 1939be9..f1bb9fb 100644
--- a/cnc25d/tests/gimbal_macro.py
+++ b/cnc25d/tests/gimbal_macro.py
@@ -63,7 +63,7 @@
 try: # when working with an installed Cnc25D package
   from cnc25d import cnc25d_api
 except:    # when working on the source files
-  import importing_cnc25d # give access to the cnc25d package
+  from . import importing_cnc25d # give access to the cnc25d package
   from cnc25d import cnc25d_api
 cnc25d_api.importing_freecad()
 
diff --git a/cnc25d/tests/juergen_four_gearwheel_test.py b/cnc25d/tests/juergen_four_gearwheel_test.py
index 54b845a..f3d328d 100644
--- a/cnc25d/tests/juergen_four_gearwheel_test.py
+++ b/cnc25d/tests/juergen_four_gearwheel_test.py
@@ -32,7 +32,7 @@
 try: # when working with an installed Cnc25D package
   from cnc25d import cnc25d_api
 except:    # when working on the source files
-  import importing_cnc25d # give access to the cnc25d package
+  from . import importing_cnc25d # give access to the cnc25d package
   from cnc25d import cnc25d_api
 #cnc25d_api.importing_freecad()
 #print("FreeCAD.Version:", FreeCAD.Version())
@@ -43,6 +43,7 @@
 from cnc25d import cnc25d_design
 #
 import Part
+import six
 
     
 ################################################################
@@ -155,7 +156,7 @@ def center_position(ai_center_diameter, ai_pd1, ai_pd2, ai_pd3, ai_pd4, ai_optio
   l13_min = max(l12, l23, l34, l41)
   l13_max = min(l12+l23, l34+l41)
   l13 = (l13_min+l13_max)/2.0
-  print("l13: min {:0.3f}  max {:0.3f} l13 {:0.3f}".format(l13_min, l13_max, l13))
+  six.print_(("l13: min {:0.3f}  max {:0.3f} l13 {:0.3f}".format(l13_min, l13_max, l13)))
   # c1
   c1x = 0.0
   c1y = 0.0
@@ -181,7 +182,7 @@ def center_position(ai_center_diameter, ai_pd1, ai_pd2, ai_pd3, ai_pd4, ai_optio
   c3: x {:0.3f}  y {:0.3f}
   c4: x {:0.3f}  y {:0.3f}
   """.format(c1x, c1y, c2x, c2y, c3x, c3y, c4x, c4y)
-  print("{:s}".format(info_txt))
+  six.print_(("{:s}".format(info_txt)))
   ### dxf
   # plank outline 
   smooth_radius = 40
diff --git a/cnc25d/tests/low_torque_transmission_macro.py b/cnc25d/tests/low_torque_transmission_macro.py
index 1cd4981..6d315a7 100644
--- a/cnc25d/tests/low_torque_transmission_macro.py
+++ b/cnc25d/tests/low_torque_transmission_macro.py
@@ -41,7 +41,7 @@
 try: # when working with an installed Cnc25D package
   from cnc25d import cnc25d_api
 except:    # when working on the source files
-  import importing_cnc25d # give access to the cnc25d package
+  from . import importing_cnc25d # give access to the cnc25d package
   from cnc25d import cnc25d_api
 cnc25d_api.importing_freecad()
 
diff --git a/cnc25d/tests/motor_lid_macro.py b/cnc25d/tests/motor_lid_macro.py
index 14f483f..de52603 100644
--- a/cnc25d/tests/motor_lid_macro.py
+++ b/cnc25d/tests/motor_lid_macro.py
@@ -63,7 +63,7 @@
 try: # when working with an installed Cnc25D package
   from cnc25d import cnc25d_api
 except:    # when working on the source files
-  import importing_cnc25d # give access to the cnc25d package
+  from . import importing_cnc25d # give access to the cnc25d package
   from cnc25d import cnc25d_api
 cnc25d_api.importing_freecad()
 
diff --git a/cnc25d/tests/simple_cnc25d_api_macro.py b/cnc25d/tests/simple_cnc25d_api_macro.py
index fa3ca47..037f1d1 100644
--- a/cnc25d/tests/simple_cnc25d_api_macro.py
+++ b/cnc25d/tests/simple_cnc25d_api_macro.py
@@ -72,7 +72,7 @@
 try: # when working with an installed Cnc25D package
   from cnc25d import cnc25d_api
 except:    # when working on the source files
-  import importing_cnc25d # give access to the cnc25d package
+  from . import importing_cnc25d # give access to the cnc25d package
   from cnc25d import cnc25d_api # import the Cnc25D API modules
 # add the FreeCAD library path to the search path
 cnc25d_api.importing_freecad()
@@ -81,6 +81,7 @@
 from FreeCAD import Base
 #
 import math # to get the pi number
+import six
 #import Tkinter # to display the outline in a small GUI
 #import svgwrite
 #from dxfwrite import DXFEngine
@@ -146,10 +147,10 @@
 cnc25d_api.write_figure_in_dxf(simple_figure, "{:s}/simple_part_mozman.dxf".format(test_output_dir))
 
 # simple_part in 3D BRep
-print("Generate {:s}/simple_part.brep".format(test_output_dir))
+six.print_(("Generate {:s}/simple_part.brep".format(test_output_dir)))
 simple_part.exportBrep("{:s}/simple_part.brep".format(test_output_dir))
 # simple_part in 2D DXF
-print("Generate {:s}/simple_part.dxf".format(test_output_dir))
+six.print_(("Generate {:s}/simple_part.dxf".format(test_output_dir)))
 # slice simple_part in the XY plan at a height of simple_extrude_height/2
 cnc25d_api.export_to_dxf(simple_part, Base.Vector(0,0,1), simple_extrude_height/2, "{:s}/simple_part.dxf".format(test_output_dir))
 
diff --git a/cnc25d/tests/split_gearwheel_macro.py b/cnc25d/tests/split_gearwheel_macro.py
index 4cbc4c4..44cb279 100644
--- a/cnc25d/tests/split_gearwheel_macro.py
+++ b/cnc25d/tests/split_gearwheel_macro.py
@@ -63,7 +63,7 @@
 try: # when working with an installed Cnc25D package
   from cnc25d import cnc25d_api
 except:    # when working on the source files
-  import importing_cnc25d # give access to the cnc25d package
+  from . import importing_cnc25d # give access to the cnc25d package
   from cnc25d import cnc25d_api
 cnc25d_api.importing_freecad()
 
diff --git a/scr/micropreprocessor.py b/scr/micropreprocessor.py
index 8d0dc09..c94d950 100755
--- a/scr/micropreprocessor.py
+++ b/scr/micropreprocessor.py
@@ -36,6 +36,7 @@
 ##########################################################################
 
 import sys, re, os
+import six
 
 micropreprocessor_usage = """
 micropreprocessor.py usage:
@@ -79,15 +80,15 @@ def process_include(ai_txt, ai_input_dir):
         #print("dbg414: include_path:", include_path)
         #if(os.path.exists(include_path)):
         try:
-          print("include file {:s}".format(include_path))
+          six.print_(("include file {:s}".format(include_path)))
           ifh = open(include_path, 'r')
           r_out_txt += ifh.read()
           ifh.close()
         except:
-          print("ERR701: Error, the file {:s} doesn't exist at line {:d}!".format(include_path, line_nb))
+          six.print_(("ERR701: Error, the file {:s} doesn't exist at line {:d}!".format(include_path, line_nb)))
           sys.exit(1)
       else:
-        print("WARN015: Warning, ambigous include statment ignore at line %d"%line_nb)
+        six.print_(("WARN015: Warning, ambigous include statment ignore at line %d"%line_nb))
         r_out_txt += i_line
     else:
       r_out_txt += i_line 
@@ -103,12 +104,12 @@ def micropreprocessor(ai_args):
   main function of micropreprocessor.py
   """
   if(len(ai_args) != 1):
-    print("ERR233: Error, micropreprocessor takes exactly one argument. %d arguments given!"%len(ai_args))
+    six.print_(("ERR233: Error, micropreprocessor takes exactly one argument. %d arguments given!"%len(ai_args)))
     print(micropreprocessor_usage)
     sys.exit(1)
   input_filename = ai_args[0]
   if(not os.path.exists(input_filename)):
-    print("ERR774: Error, the input file {:s} doesn't exist!".format(input_filename))
+    six.print_(("ERR774: Error, the input file {:s} doesn't exist!".format(input_filename)))
     sys.exit(1)
   ifh = open(input_filename, 'r')
   input_content = ifh.read()
@@ -118,7 +119,7 @@ def micropreprocessor(ai_args):
   ofh = open(output_filename, 'w')
   ofh.write(output_content)
   ofh.close()
-  print("The outfile {:s} has been writen :)".format(output_filename))
+  six.print_(("The outfile {:s} has been writen :)".format(output_filename)))
 
 ##########################################################################
 # main
diff --git a/scr/svg2txt.py b/scr/svg2txt.py
index 69cabc0..41236f3 100755
--- a/scr/svg2txt.py
+++ b/scr/svg2txt.py
@@ -39,6 +39,7 @@
 from lxml import etree
 import sys
 import re
+import six
 
 #def transform(xmlPath, xslPath):
 def transform_svg2txt(xmlPath):
@@ -65,11 +66,11 @@ def transform_svg2txt(xmlPath):
 if __name__ == '__main__':
   #print("dbg101: sys.argv:", sys.argv)
   if(len(sys.argv)==1):
-    print("%s"%help_msg)
+    six.print_(("%s"%help_msg))
     print("Nothing done!\n")
   else:
     for f_svg in sys.argv[1:]:
-      print("Processing svg file: %s"%(f_svg))
+      six.print_(("Processing svg file: %s"%(f_svg)))
       f_txt = re.sub('\.svg','', f_svg) + ".txt"
       #print(transform_svg2txt('./s0101m.mul0.xml', './tipitaka-latn.xsl'))
       txt_content = transform_svg2txt(f_svg)
@@ -78,6 +79,6 @@ def transform_svg2txt(xmlPath):
       output_file_handler = open(f_txt, 'w')
       output_file_handler.write(txt_content)
       output_file_handler.close()
-      print("and outputs text file: %s"%(f_txt))
+      six.print_(("and outputs text file: %s"%(f_txt)))