diff --git a/README b/README
index 0a10f81..e99d361 100644
--- a/README
+++ b/README
@@ -1,5 +1,5 @@
 --------------------------------------------------------------------------
-XSIM     2020/03/11 v0.19
+XSIM     2020/03/16 v0.19
 XSIMVERB 2019/12/29 v0.1a
 
   eXercise Sheets IMproved
diff --git a/code/xsim.auxfile.code.tex b/code/xsim.auxfile.code.tex
index c108eb1..2f31f20 100644
--- a/code/xsim.auxfile.code.tex
+++ b/code/xsim.auxfile.code.tex
@@ -85,14 +85,8 @@
 
 \xsim_new_aux_property:cpn {readaux}
   {
-    % can I do this with expl3?
-    % cf. issue #60
-    \InputIfFileExists { \c_sys_jobname_str .xsim }
-      { \xsim_verbose:n { Reading~ \c_sys_jobname_str .xsim } }
-      {
-        \xsim_verbose:n { \c_sys_jobname_str .xsim~ not~ found.}
-        \xsim_rerun:
-      }
+    \file_if_exist:nT { \c_sys_jobname_str .xsim }
+      { \file_input:n { \c_sys_jobname_str .xsim } }
   }
 
 % ----------------------------------------------------------------------------
@@ -106,27 +100,6 @@
     split-aux-lists .initial:n = ||
   }
 
-\cs_new_protected:Npn \__xsim_set_list_value:nn #1#2
-  {
-    \cs_undefine:c {____xsim_list_ \tl_to_str:n {#1!#2} :}
-    \cs_new:cpn {____xsim_list_ \tl_to_str:n {#1!#2} :} {}
-  }
-
-\cs_new_protected:Npn \__xsim_unset_list_value:nn #1#2
-  { \cs_undefine:c {____xsim_list_ \tl_to_str:n {#1!#2} :} }
-
-% check if in list:
-% #1: name
-% #2: entry
-\prg_new_conditional:Npnn \xsim_if_in_list:nn #1#2 {p,T,F,TF}
-  {
-    \cs_if_exist:cTF {____xsim_list_ \tl_to_str:n {#1!#2} :}
-      { \prg_return_true: }
-      { \prg_return_false: }
-  }
-\cs_generate_variant:Nn \xsim_if_in_list:nnTF {nx}
-\cs_generate_variant:Nn \xsim_if_in_list:nnT  {nx}
-
 % define list:
 % #1: name
 \cs_new_protected:Npn \xsim_new_list:n #1
@@ -142,7 +115,7 @@
               \l__xsim_split_aux_lists_str
               { \tl_to_str:n {##1} }
             \seq_map_inline:cn {g__xsim_list_#1_seq}
-              { \__xsim_set_list_value:nn {#1} {####1} }
+              { \xsim_attribute_set:nnn {#1} {####1} {} }
           }
       }
     \xsim_at_begin_document:n
@@ -165,6 +138,7 @@
   {
     \xsim_verbose:n { Adding~ `#2'~ to~ list~ `#1'. }
     \str_set:Nn \l__xsim_tmpa_str {#2}
+    \xsim_attribute_set:nnn {#1} {#2} {}
     \seq_if_in:cVT {g__xsim_list_#1_seq} \l__xsim_tmpa_str
       { \seq_gremove_all:cV {g__xsim_list_#1_seq} \l__xsim_tmpa_str }
     \seq_gput_right:cV {g__xsim_list_#1_seq} \l__xsim_tmpa_str
@@ -177,7 +151,8 @@
   {
     \xsim_verbose:n { Removing~ `#2'~ to~ list~ `#1'. }
     \str_set:Nn \l__xsim_tmpa_str {#2}
-    \__xsim_unset_list_value:nn {#1} {#2}
+    \xsim_attribute_if_set:nnT {#1} {#2}
+      { \xsim_attribute_unset:nn {#1} {#2} }
     \seq_if_in:cVT {g__xsim_list_#1_seq} \l__xsim_tmpa_str
       { \seq_gremove_all:cV {g__xsim_list_#1_seq} \l__xsim_tmpa_str }
     \seq_if_in:cVT {g__xsim_list_recorded#1_seq} \l__xsim_tmpa_str
@@ -185,6 +160,18 @@
   }
 \cs_generate_variant:Nn \xsim_remove_from_list:nn {nx}
 
+% check if in list:
+% #1: name
+% #2: entry
+\prg_new_conditional:Npnn \xsim_if_in_list:nn #1#2 {p,T,F,TF}
+  {
+    \xsim_attribute_if_set:nnTF {#1} {#2}
+      { \prg_return_true: }
+      { \prg_return_false: }
+  }
+\cs_generate_variant:Nn \xsim_if_in_list:nnTF {nx}
+\cs_generate_variant:Nn \xsim_if_in_list:nnT  {nx}
+
 \bool_new:N \l__xsim_empty_lists_bool
 \cs_new_protected:Npn \xsim_empty_lists:
   { \bool_set_true:N \l__xsim_empty_lists_bool }
@@ -231,4 +218,4 @@
 
 % ----------------------------------------------------------------------------
 \file_input_stop:
-2020/03/10 - performance optimizations
+2020/03/16 - performance optimizations
diff --git a/code/xsim.base.code.tex b/code/xsim.base.code.tex
index fbaf246..3bcfb21 100644
--- a/code/xsim.base.code.tex
+++ b/code/xsim.base.code.tex
@@ -53,7 +53,7 @@
 \cs_generate_variant:Nn \seq_if_in:NnT      {cV}
 \cs_generate_variant:Nn \seq_gremove_all:Nn {cV}
 \cs_generate_variant:Nn \seq_gput_right:Nn  {cV}
-\cs_generate_variant:Nn \prop_put:Nnn       {cxx}
+\cs_generate_variant:Nn \prop_put:Nnn       {Nnx,cxx}
 \cs_generate_variant:Nn \prop_gput:Nnn      {Nx,Nnx,cxx}
 \cs_generate_variant:Nn \prop_item:Nn       {c}
 \cs_generate_variant:Nn \prop_if_in:NnTF    {Nx}
@@ -68,12 +68,11 @@
 \cs_generate_variant:Nn \tl_if_eq:nnTF      {ff,V}
 \cs_generate_variant:Nn \tl_if_eq:nnT       {x}
 \cs_generate_variant:Nn \tl_if_blank:nTF    {f}
-\cs_generate_variant:Nn \tl_set_rescan:Nnn  {Nnn}
+\cs_generate_variant:Nn \tl_set_rescan:Nnn  {Nnx}
 \cs_generate_variant:Nn \iow_now:Nn         {NV}
 \cs_generate_variant:Nn \str_if_eq:nnT      {x}
 \cs_generate_variant:Nn \str_if_eq:nnF      {xx}
 \cs_generate_variant:Nn \str_if_eq_p:nn     {e}
-\cs_generate_variant:Nn \iow_now:Nn         {NV}
 \cs_generate_variant:Nn \prg_new_protected_conditional:Npnn {c}
 
 % ----------------------------------------------------------------------------
@@ -136,6 +135,13 @@
       { \prg_return_false: }
   }
 
+\prg_new_conditional:Npnn \xsim_if_debug: {p,T,F,TF}
+  {
+    \bool_if:NTF \g__xsim_debug_bool
+      { \prg_return_true: }
+      { \prg_return_false: }
+  }
+
 \prg_new_conditional:Npnn \xsim_if_package_blank: {p,T,F,TF}
   {
     \bool_if:NTF \g__xsim_blank_bool
@@ -198,8 +204,78 @@
 
 % ----------------------------------------------------------------------------
 
+\msg_new:nnn {xsim} {attribute-not-set}
+  { The~ attribute~ `#1'~ is~ not~ set~ \msg_line_context: }
+
+% #1: attribute
+% #2: item
+\cs_new_protected:Npn \xsim_attribute_new:nn #1#2
+  { \cs_new:cpn {____xsim_attribute_ \tl_to_str:n {#1::#2} :} {} }
+  
+% #1: attribute
+% #2: item
+% #3: value
+\cs_new_protected:Npn \xsim_attribute_set:nnn #1#2#3
+  { \cs_gset:cpx {____xsim_attribute_ \tl_to_str:n {#1::#2} :} { \exp_not:n {#3} } }
+\cs_generate_variant:Nn \xsim_attribute_set:nnn {nnx}
+
+% #1: attribute
+% #2: item
+\cs_new_protected:Npn \xsim_attribute_unset:nn #1#2
+  { \cs_undefine:c {____xsim_attribute_ \tl_to_str:n {#1::#2} :} }
+  
+% #1: attribute
+% #2: item
+\cs_new:Npn \xsim_attribute_get:nn #1#2
+  {
+    \cs_if_exist:cTF {____xsim_attribute_ \tl_to_str:n {#1::#2} :}
+      { \use:c {____xsim_attribute_ \tl_to_str:n {#1::#2} :} }
+      {
+        \xsim_if_debug:T
+          { \msg_expandable_error:nnn {xsim} {attribute-not-set} {#1::#2} }
+      }
+  }
+
+% #1: attribute
+% #2: item
+\prg_new_conditional:Npnn \xsim_attribute_if_set:nn #1#2 {p,T,F,TF}
+  {
+    \cs_if_exist:cTF {____xsim_attribute_ \tl_to_str:n {#1::#2} :}
+      { \prg_return_true: }
+      { \prg_return_false: }
+  }
+
+% #1: attribute
+% #2: item
+\prg_new_conditional:Npnn \__xsim_if_attribute_eq:nnn #1#2#3 {p,T,F,TF}
+  {
+    \tl_if_eq:enTF
+      { \use:c {____xsim_attribute_ \tl_to_str:n {#1::#2} :} }
+      {#3}
+      { \prg_return_true: }
+      { \prg_return_false: }
+  }
+
+% #1: attribute
+% #2: item
+\cs_new_protected:Npn \xsim_attribute_show:nn #1#2
+  {
+    \iow_term:n {}
+    \iow_term:x
+      {
+        \c_space_tl \c_space_tl
+        \tl_to_str:n {#1::#2}
+        \c_space_tl \c_space_tl == \c_space_tl \c_space_tl
+        {\use:c {____xsim_attribute_ \tl_to_str:n {#1::#2} :}}
+      }
+    \iow_term:n {}
+  }
+
+% ----------------------------------------------------------------------------
+
 \cs_new_protected:Npn \xsim_setup:n #1
   { \keys_set:nn {xsim} {#1} }
 
 % ----------------------------------------------------------------------------
 \file_input_stop:
+2020/03/15 - alternative to property lists for performance optimizations
diff --git a/code/xsim.collections.code.tex b/code/xsim.collections.code.tex
index b85ffa2..5aeb6e2 100644
--- a/code/xsim.collections.code.tex
+++ b/code/xsim.collections.code.tex
@@ -29,7 +29,7 @@
 
 \xsim_load_modules:n {base}
 
-\prop_new:N \g__xsim_collections_prop
+\seq_new:N \g__xsim_collections_seq
 \bool_new:N \l____xsim_active_bool
 
 \msg_new:nnn {xsim} {collection-exists}
@@ -66,7 +66,8 @@
       { \msg_error:nnn {xsim} {collection-exists} {#1} }
       {
         \xsim_verbose:n { Declaring~ new~ collection~ `#1'. }
-        \prop_gput:Nnn \g__xsim_collections_prop {#1} { \c_false_bool }
+        \xsim_attribute_set:nnn {collection} {#1} { \c_false_bool }
+        \seq_gput_right:Nn \g__xsim_collections_seq {#1}
         \xsim_new_list:n {collection:#1}
         \seq_new:c {g__xsim_collection_#1_seq}
       }
@@ -75,7 +76,7 @@
 % #1: collection name
 \prg_new_conditional:Npnn \xsim_if_collection_exist:n #1 {T,F,TF}
   {
-    \prop_if_in:NnTF \g__xsim_collections_prop {#1}
+    \seq_if_in:NnTF \g__xsim_collections_seq {#1}
       { \prg_return_true: }
       { \prg_return_false: }
   }
@@ -83,7 +84,9 @@
 \xsim_new_collection:n {all~ exercises}
 
 \cs_new_protected:Npn \xsim_foreach_collection:n #1
-  { \prop_map_inline:Nn \g__xsim_collections_prop {#1} }
+  { \seq_map_inline:Nn \g__xsim_collections_seq {#1} }
+
+\cs_new:Npn \xsim_collection_map_break: { \seq_map_break: }
 
 % ----------------------------------------------------------------------------
 
@@ -96,7 +99,7 @@
       }
       {
         \xsim_verbose:n { Activating~ collection~ `#1'. }
-        \prop_gput:Nnn \g__xsim_collections_prop {#1} { \c_true_bool }
+        \xsim_attribute_set:nnn {collection} {#1} { \c_true_bool }
         \tl_if_blank:nF {#2}
           { \seq_gput_right:cn {g__xsim_collection_#1_seq} {#2} }
       }
@@ -111,8 +114,8 @@
     \xsim_if_collection_active:nTF {#1}
       {
         \xsim_verbose:n { Deactivating~ collection~ `#1'. }
-        \prop_gput:Nnn \g__xsim_collections_prop {#1} { \c_false_bool }
-        \seq_gclear:c {g__xsim_collection_#1_seq}
+        \xsim_attribute_set:nnn {collection} {#1} { \c_false_bool }
+        % \seq_gclear:c {g__xsim_collection_#1_seq}
       }
       { % TODO: warning
       }
@@ -161,7 +164,7 @@
     \seq_if_empty:cTF {g__xsim_collection_#1_seq}
       { \__xsim_add_to_collection:nnn {#1} {#2} {#3} }
       {
-        \seq_if_in:cnT {g__xsim_collection_#1_seq} {#2}
+        \seq_if_in:cnF {g__xsim_collection_#1_seq} {#2}
           { \__xsim_add_to_collection:nnn {#1} {#2} {#3} }
       }
   }
@@ -349,11 +352,11 @@
 % #1: collection name
 \prg_new_conditional:Npnn \xsim_if_collection_active:n #1 {T,F,TF}
   {
-    \prop_if_in:NnTF \g__xsim_collections_prop {#1}
+    \xsim_attribute_if_set:nnTF {collection} {#1}
       {
         \bool_lazy_and:nnTF
           { !\str_if_eq_p:en {#1} {all~ exercises} }
-          { \prop_item:Nn \g__xsim_collections_prop {#1} }
+          { \xsim_attribute_get:nn {collection} {#1} }
           { \prg_return_true: }
           { \prg_return_false: }
       }
@@ -363,12 +366,12 @@
 \prg_new_protected_conditional:Npnn \xsim_if_collections_active: {T,F,TF}
   {
     \bool_set_false:N \l____xsim_active_bool
-    \prop_map_inline:Nn \g__xsim_collections_prop
+    \xsim_foreach_collection:n
       {
         \xsim_if_collection_active:nT {##1}
           {
             \bool_set_true:N \l____xsim_active_bool
-            \prop_map_break:
+            \xsim_collection_map_break:
           }
       }
     \bool_if:NTF \l____xsim_active_bool
@@ -381,7 +384,7 @@
 \xsim_at_begin_document:n
   {
     \xsim_get_collections:
-    \xsim_activate_collection:n   {all~ exercises}
+    \xsim_activate_collection:n {all~ exercises}
   }
 
 \xsim_at_end_document:n
diff --git a/code/xsim.environments.code.tex b/code/xsim.environments.code.tex
index 89f65c0..ca5f76f 100644
--- a/code/xsim.environments.code.tex
+++ b/code/xsim.environments.code.tex
@@ -55,9 +55,6 @@
 \tl_new:N  \ExerciseText
 \tl_set:Nn \ExerciseText { \msg_error:nn {xsim} {only-in-solution} }
 
-\cs_generate_variant:Nn \tl_set_rescan:Nnn {Nnx}
-\cs_generate_variant:Nn \prop_put:Nnn {Nnx}
-
 % ----------------------------------------------------------------------------
 
 \msg_new:nnn {xsim} {only-in-solution}
diff --git a/code/xsim.exercises.code.tex b/code/xsim.exercises.code.tex
index 608eacc..9ba49c6 100644
--- a/code/xsim.exercises.code.tex
+++ b/code/xsim.exercises.code.tex
@@ -33,10 +33,10 @@
     collections
   }
 
+\seq_new:N  \l__xsim_exercise_types_seq
 \seq_new:N  \l__xsim_parameters_seq
 \seq_new:N  \l__xsim_fixed_parameters_seq
 \seq_new:N  \l__xsim_mandatory_parameters_seq
-\prop_new:N \g__xsim_exercises_prop
 \int_gzero_new:N  \g_xsim_id_int
 \int_gzero_new:N  \g_xsim_max_id_int
 \tl_new:N   \l_xsim_current_id_tl
@@ -78,17 +78,19 @@
 % #2: parameter
 % #3: value
 \cs_new_protected:Npn \__xsim_set_parameter_type:nnn #1#2#3
-  { \cs_set:cpn {__xsim_parameter_ \tl_to_str:n {#1!#2} :} {#3} }
+  { \xsim_attribute_set:nnn {#1} {#2} {#3} }
 
 % #1: type
 % #2: parameter
 \cs_new:Npn \xsim_get_parameter:nn #1#2
-  { \cs_if_exist_use:c {__xsim_parameter_ \tl_to_str:n {#1!#2} :} }
+  { \xsim_attribute_get:nn {#1} {#2} }
 \cs_generate_variant:Nn \xsim_get_parameter:nn {o}
 
+% #1: type
+% #2: parameter
 \prg_new_conditional:Npnn \xsim_if_parameter_set:nn #1#2 {p,T,F,TF}
   {
-    \cs_if_exist:cTF {__xsim_parameter_ \tl_to_str:n {#1!#2} :}
+    \xsim_attribute_if_set:nnTF {#1} {#2}
       { \prg_return_true: }
       { \prg_return_false: }
   }
@@ -162,7 +164,7 @@
 % #1: type
 \prg_new_conditional:Npnn \xsim_if_type:n #1 {p,T,F,TF}
   {
-    \cs_if_exist:cTF {--__xsim_#1_type_parameter:}
+    \seq_if_in:NnTF \l__xsim_exercise_types_seq {#1}
       { \prg_return_true: }
       { \prg_return_false: }
   }
@@ -282,7 +284,7 @@
 
 \cs_new_protected:Npn \xsim_define_exercise_type_variables:n #1
   {
-    % \prop_new:c {l__xsim_#1_type_parameters_prop}
+    \seq_put_right:Nn \l__xsim_exercise_types_seq {#1}
     \bool_new:c {l__xsim_#1_solution_print_bool}
     \bool_new:c {l__xsim_#1_exercise_print_bool}
     \bool_new:c {l__xsim_#1_exercise_use_bool}
@@ -631,4 +633,4 @@
 2020/02/21 - implement issue #2
 2020/03/01 - add options `<ex-env>s/name` and `<sol-env>/name`
 2020/03/07 - set \ExerciseID and \ExerciseType only locally
-2020/03/11 - performance optimizations
+2020/03/14 - performance optimizations
diff --git a/code/xsim.modules.code.tex b/code/xsim.modules.code.tex
index bc82dcb..51b595f 100644
--- a/code/xsim.modules.code.tex
+++ b/code/xsim.modules.code.tex
@@ -87,7 +87,7 @@
           { \msg_error:nnn {xsim} {module-missing} {#1} }
       }
   }
-\cs_generate_variant:Nn \__xsim_load_module:n { V }
+\cs_generate_variant:Nn \__xsim_load_module:n {V}
 
 \prg_new_conditional:Npnn \xsim_if_module_loaded:n #1 {p,T,F,TF}
   {
@@ -163,8 +163,6 @@
     \tl_set:Nx \l__xsim_module_version_tl {\c_xsim_version_tl}
   }
 
-\cs_generate_variant:Nn \str_if_eq:nnF {xx}
-
 \cs_new_protected:Npn \__xsim_module:nn #1#2
   {
     \seq_gput_right:Nn \g__xsim_loaded_modules_seq {#1}
diff --git a/code/xsim.properties.code.tex b/code/xsim.properties.code.tex
index 28ec7d9..12a6fdf 100644
--- a/code/xsim.properties.code.tex
+++ b/code/xsim.properties.code.tex
@@ -66,26 +66,8 @@
 % #1: type
 % #2: id
 % #3: property
-\cs_new_protected:Npn \____xsim_define_property:nnn #1#2#3
-  { \cs_new:cpn {____xsim_property_ \tl_to_str:n {#1!#2!#3} :} {} }
-
-% #1: type
-% #2: id
-% #3: property
-\cs_new_protected:Npn \____xsim_set_property:nnnn #1#2#3#4
-  { \cs_set:cpn {____xsim_property_ \tl_to_str:n {#1!#2!#3} :} {#4} }
-
-% #1: type
-% #2: id
-% #3: property
-\cs_new_protected:Npn \____xsim_unset_property:nnn #1#2#3
-  { \cs_undefine:c {____xsim_property_ \tl_to_str:n {#1!#2!#3} :} }
-
-% #1: type
-% #2: id
-% #3: property
-\cs_new:Npn \____xsim_get_property:nnn #1#2#3
-  { \cs_if_exist_use:c {____xsim_property_ \tl_to_str:n {#1!#2!#3} :} }
+\cs_new_protected:Npn \__xsim_define_property:nnn #1#2#3
+  { \xsim_attribute_new:nn {#1!#2} {#3} }
 
 % check if property is set:
 % #1: type
@@ -93,7 +75,7 @@
 % #3: property
 \prg_new_conditional:Npnn \xsim_if_property_set:nnn #1#2#3 {p,T,F,TF}
   {
-    \cs_if_exist:cTF {____xsim_property_ \tl_to_str:n {#1!#2!#3} :}
+    \xsim_attribute_if_set:nnTF {#1!#2} {#3}
       { \prg_return_true: }
       { \prg_return_false: }
   }
@@ -132,8 +114,6 @@
                 \seq_if_in:NnF \l__xsim_unique_properties_seq {#4}
                   { \seq_put_right:Nn \l__xsim_unique_properties_seq {#4} }
                 \seq_new:c {l__xsim_unique_property_#4_values_seq}
-                \prop_new:c {g__xsim_property_#4_type_prop}
-                \prop_new:c {g__xsim_property_#4_id_prop}
               }
               { \xsim_verbose:n { Declaring~ new~ exercise~ property~ `#4'. } }
           }
@@ -259,6 +239,17 @@
     \__xsim_set_property_reverse:nnnn {#1} {#2} {#3} {#4}
   }
 
+% #1: type
+% #2: id
+% #3: property
+\cs_new_protected:Npn \__xsim_unset_property:nnn #1#2#3
+  {
+    \xsim_verbose:n
+      { Un-setting~ property~ `#3'~ of~ exercise~ type~ `#1'~ id~ `#2'. }
+    \__xsim_remove_property_from_list:nnn {#1} {#2} {#3}
+    \xsim_attribute_unset:nn {#1!#2} {#3}
+  }
+
 % #1: type
 % #2: id
 % #3: property
@@ -284,7 +275,7 @@
     \__xsim_remove_property_from_list:nnn {#1} {#2} {#3}
     % now we can safely add the property to the list:
     \xsim_add_to_list:nn {#3} {#1-#2=={#4}}
-    \____xsim_set_property:nnnn {#1} {#2} {#3} {#4}
+    \xsim_attribute_set:nnn {#1!#2} {#3} {#4}
   }
 \cs_generate_variant:Nn \__xsim_set_property:nnnn {VVnV}
 
@@ -294,19 +285,8 @@
 % #4: value
 \cs_new_protected:Npn \__xsim_set_property_reverse:nnnn #1#2#3#4
   {
-    \prop_gput:cnn {g__xsim_property_#3_type_prop} {#4} {#1}
-    \prop_gput:cnn {g__xsim_property_#3_id_prop} {#4} {#2}
-  }
-
-% #1: type
-% #2: id
-% #3: property
-\cs_new_protected:Npn \__xsim_unset_property:nnn #1#2#3
-  {
-    \xsim_verbose:n
-      { Un-setting~ property~ `#3'~ of~ exercise~ type~ `#1'~ id~ `#2'. }
-    \__xsim_remove_property_from_list:nnn {#1} {#2} {#3}
-    \____xsim_unset_property:nnn {#1} {#2} {#3}
+    \xsim_attribute_set:nnn {type:#3} {#4} {#1}
+    \xsim_attribute_set:nnn {id:#3} {#4} {#2}
   }
 
 \tl_set:Nx \l__xsim_tmpa_tl { \cs_to_str:N \{ }
@@ -324,6 +304,9 @@
     \str_set:Nn #3 {#6}
   }
 
+% #1: type
+% #2: id
+% #3: property
 \cs_new_protected:Npn \__xsim_remove_property_from_list:nnn #1#2#3
   {
     % make sure we change the entry if it has been recorded previously:
@@ -391,20 +374,19 @@
 
 % ----------------------------------------------------------------------------
 
-\prop_new:N \g__xsim_property_aliases_prop
 \seq_new:N \g__xsim_property_aliases_seq
 
 % #1: property
 \prg_new_conditional:Npnn \xsim_if_property_alias:n #1 {p,T,F,TF}
   {
-    \prop_if_in:NnTF \g__xsim_property_aliases_prop {#1}
+    \seq_if_in:NnTF \g__xsim_property_aliases_seq {#1}
       { \prg_return_true: }
       { \prg_return_false: }
   }
 
 % #1: property
 \cs_new:Npn \xsim_property_alias:n #1
-  { \prop_item:Nn \g__xsim_property_aliases_prop {#1} }
+  { \xsim_attribute_get:nn {#1} {alias} }
 
 % #1: property
 % #2: alias of
@@ -414,8 +396,7 @@
       {
         \xsim_verbose:n
           { Making~ property~ `#1'~ an~ alias~ of~ property~ `#2'. }
-        \prop_new:c {g__xsim_property_#1_alias_prop}
-        \prop_gput:Nnn \g__xsim_property_aliases_prop {#1} {#2}
+        \xsim_attribute_set:nnn {#1} {alias} {#2}
         \seq_gput_right:Nn \g__xsim_property_aliases_seq {#1}
       }
   }
@@ -440,8 +421,7 @@
     \xsim_if_property_set:nnxT {#1} {#2}
       { \xsim_property_alias:n {#3} }
       {
-        \prop_gput:cxx {g__xsim_property_#3_alias_prop}
-          { \xsim_get_property:nnn {#1} {#2} {#3} }
+        \xsim_attribute_set:nnx {alias:#1!#2} {#3}
           {
             \xsim_get_property:nnf {#1} {#2}
               { \xsim_property_alias:n {#3} }
@@ -465,18 +445,18 @@
 % #2: id
 % #3: property
 \cs_new:Npn \xsim_get_property:nnn #1#2#3
-  { \____xsim_get_property:nnn {#1} {#2} {#3} }
+  { \xsim_attribute_get:nn {#1!#2} {#3} }
 \cs_generate_variant:Nn \xsim_get_property:nnn {nx,ne,xx,oo,no,nnf,nf,oof,nV}
 
 % #1: property
 % #2: value
 \cs_new:Npn \xsim_get_id_for_property:nn #1#2
-  { \prop_item:cn {g__xsim_property_#1_id_prop} {#2} }
+  { \xsim_attribute_get:nn {id:#1} {#2} }
 
 % #1: property
 % #2: value
 \cs_new:Npn \xsim_get_type_for_property:nn #1#2
-  { \prop_item:cn {g__xsim_property_#1_type_prop} {#2} }
+  { \xsim_attribute_get:nn {type:#1} {#2} }
 \cs_generate_variant:Nn \xsim_get_type_for_property:nn {nV}
 
 % #1: id or ID
@@ -557,4 +537,4 @@
 % ----------------------------------------------------------------------------
 \file_input_stop:
 2020/03/09 - add meaningful error if a comma is missing in the property list
-2020/03/10 - performance optimizations
+2020/03/14 - performance optimizations
diff --git a/code/xsim.random.code.tex b/code/xsim.random.code.tex
index 4229637..cdd23e3 100644
--- a/code/xsim.random.code.tex
+++ b/code/xsim.random.code.tex
@@ -94,7 +94,7 @@
 % #1: random list id
 % #2: sequence variable to set list from
 \cs_new_protected:Npn \xsim_save_random_list:nN #1#2
-  {
+  { % TODO: check if #2 is empty
     \xsim_random_list_if_exist:nF {#1}
       { \xsim_new_random_list:n {#1} }
     \xsim_random_list_if_empty:nT {#1}
@@ -103,6 +103,7 @@
     \xsim_add_property_to_aux:nn {random}
       { {#1} { \xsim_use_random_list:nn {#1} {,} } }
   }
+\cs_generate_variant:Nn \xsim_save_random_list:nN {x}
 
 % #1: collection
 % #2: number of items
@@ -142,7 +143,7 @@
               { \l__xsim_tmpa_int }
               \g__xsim_random_list_seq
           }
-        \xsim_save_random_list:nN
+        \xsim_save_random_list:xN
           { \int_to_alph:n { \g__xsim_random_list_id_int } }
           \g__xsim_random_list_seq
         \xsim_verbose:x
diff --git a/code/xsim.solutions.code.tex b/code/xsim.solutions.code.tex
index 2666d1a..0a69194 100644
--- a/code/xsim.solutions.code.tex
+++ b/code/xsim.solutions.code.tex
@@ -35,7 +35,7 @@
 \bool_new:N \l__xsim_printsolutions_conditions_bool
 \bool_new:N \l____xsim_this_condition_bool
 
-\prop_new:N \l__xsim_printsolutions_conditions_prop
+\seq_new:N \l__xsim_printsolutions_conditions_seq
 
 \int_new:N  \l_xsim_printsolutions_section_int
 \int_new:N  \l_xsim_printsolutions_chapter_int
@@ -194,7 +194,8 @@
 \cs_new_protected:Npn \xsim_declare_printsolutions_condition:nn #1#2
   {
     \bool_new:c {l__xsim_printsolutions_#1_condition_bool}
-    \prop_put:Nnn \l__xsim_printsolutions_conditions_prop {#1} {#2}
+    \seq_put_right:Nn \l__xsim_printsolutions_conditions_seq {#1}
+    \xsim_attribute_set:nnn {printsolutions} {#1} {#2}
   }
 
 \prg_new_conditional:Npnn \__xsim_printsolutions_if_condition:n #1 {T,F,TF}
@@ -218,11 +219,12 @@
   {
     \__xsim_printsolutions_condition_false:
     \bool_set_true:N \l____xsim_this_condition_bool
-    \prop_map_inline:Nn \l__xsim_printsolutions_conditions_prop
+    \seq_map_inline:Nn \l__xsim_printsolutions_conditions_seq
       {
         \bool_if:cT {l__xsim_printsolutions_##1_condition_bool}
           {
-            ##2 {#1} {#2} {#3}
+            \xsim_attribute_get:nn {printsolutions} {##1}
+              {#1} {#2} {#3}
               { }
               { \bool_set_false:N \l____xsim_this_condition_bool }
           }
@@ -409,3 +411,4 @@
 \file_input_stop:
 
 2020/02/23 - resolve bug #56
+2020/03/16 - performance optimizations
diff --git a/code/xsim.sty b/code/xsim.sty
index dc5b761..79ab9b4 100644
--- a/code/xsim.sty
+++ b/code/xsim.sty
@@ -29,7 +29,7 @@
 
 \ExplSyntaxOn
 
-\tl_const:Nn \c_xsim_date_tl                 {2020/03/11}
+\tl_const:Nn \c_xsim_date_tl                 {2020/03/16}
 \tl_const:Nn \c_xsim_version_major_number_tl {0}
 \tl_const:Nn \c_xsim_version_minor_number_tl {19}
 \tl_const:Nn \c_xsim_version_subrelease_tl   {}
@@ -57,6 +57,7 @@
 
 \bool_new:N \g__xsim_final_bool
 \bool_new:N \g__xsim_verbose_bool
+\bool_new:N \g__xsim_debug_bool
 \bool_new:N \g_xsim_clear_aux_bool
 \bool_new:N \g__xsim_write_to_file_bool
 \bool_new:N \g_xsim_use_aux_bool
@@ -67,6 +68,7 @@
   {
     final     .bool_gset:N         = \g__xsim_final_bool ,
     verbose   .bool_gset:N         = \g__xsim_verbose_bool ,
+    debug     .bool_gset:N         = \g__xsim_debug_bool ,
     clear-aux .bool_gset:N         = \g_xsim_clear_aux_bool ,
     no-files  .bool_gset_inverse:N = \g__xsim_write_to_file_bool ,
     no-files  .initial:n           = false ,
@@ -82,6 +84,7 @@
   {
     final     .code:n = \msg_error:nnn {xsim} {load-time-option} {final} ,
     verbose   .code:n = \msg_error:nnn {xsim} {load-time-option} {verbose} ,
+    debug     .code:n = \msg_error:nnn {xsim} {load-time-option} {debug} ,
     clear-aux .code:n = \msg_error:nnn {xsim} {load-time-option} {clear-aux} ,
     no-files  .code:n = \msg_error:nnn {xsim} {load-time-option} {no-files} ,
     use-aux   .code:n = \msg_error:nnn {xsim} {load-time-option} {use-aux} ,
diff --git a/code/xsim.templates.code.tex b/code/xsim.templates.code.tex
index 7644074..35bd173 100644
--- a/code/xsim.templates.code.tex
+++ b/code/xsim.templates.code.tex
@@ -36,11 +36,21 @@
     I~ am~ using~ the~ template~ `#3'~ (template~ type~ `#1')~ instead.
   }
 
+\msg_new:nnn {xsim} {template-type-exists}
+  { The~ template~ type~ `#1'~ already~ exists~ \msg_line_context: }
+
+% ----------------------------------------------------------------------------
+\seq_new:N \l__xsim_template_types_seq
+
 % #1: template type
 \cs_new_protected:Npn \xsim_new_template_type:n #1
   {
-    \xsim_verbose:n { Declaring~ new~ template~ type~ `#1'. }
-    \prop_new:c {l__xsim_#1_template_prop}
+    \seq_if_in:NnTF \l__xsim_template_types_seq {#1}
+      { \msg_error:nnn {xsim} {template-type-exists} {#1} }
+      {
+        \xsim_verbose:n { Declaring~ new~ template~ type~ `#1'. }
+        \seq_put_right:Nn \l__xsim_template_types_seq {#1}
+      }
   }
 
 % #1: template type
@@ -50,12 +60,12 @@
   {
     \xsim_verbose:n { Defining~ new~ template~ `#2'~ of~ type~ `#1'. }
     \tl_new:c {l__xsim_template_#1_#2_setup_tl}
-    \prop_put:cnn {l__xsim_#1_template_prop} {#2} {#3}
+    \xsim_attribute_set:nnn {template::#1} {#2} {#3}
   }
 
 \prg_new_conditional:Npnn \xsim_if_template_exist:nn #1#2 {T,F,TF}
   {
-    \prop_if_in:cnTF {l__xsim_#1_template_prop} {#2}
+    \xsim_attribute_if_set:nnTF {template::#1} {#2}
       { \prg_return_true: }
       { \prg_return_false: }
   }
@@ -63,7 +73,7 @@
 % #1: template type
 % #2: template name
 \cs_new_protected:Npn \__xsim_get_template:nn #1#2
-  { \prop_item:cn {l__xsim_#1_template_prop} {#2} }
+  { \xsim_attribute_get:nn {template::#1} {#2} }
 
 % ----------------------------------------------------------------------------
 
diff --git a/doc/examples/xsim.collections.tex b/doc/examples/xsim.collections.tex
index beca191..990391e 100644
--- a/doc/examples/xsim.collections.tex
+++ b/doc/examples/xsim.collections.tex
@@ -5,7 +5,7 @@
 \DeclareExerciseCollection{foo-medium}
 \DeclareExerciseTagging{difficulty}
 
-\usepackage{filecontents,lipsum}
+\usepackage{lipsum}
 \begin{filecontents*}{foo.tex}
 \begin{exercise}[difficulty=easy,points=1]
   foo one \lipsum[4]
diff --git a/doc/xsim-manual.cls b/doc/xsim-manual.cls
index 1209f2f..e40029d 100644
--- a/doc/xsim-manual.cls
+++ b/doc/xsim-manual.cls
@@ -492,7 +492,7 @@
 \tcbset{enhanced,enhanced jigsaw}
 
 \newtcolorbox{bewareofthedog}{
-  colback  = white ,
+  colback  = red!2!white!98 ,
   colframe = red ,
   underlay={%
     \path[draw=none]
@@ -595,21 +595,7 @@
         \cnltx@after@example@hook
       }%
     \endcnltxcode
-  }\newcommand*\xsimman@readoptions[1]{%
-  \catcode`\^^M=13
-  \xsimman@read@options{#1}%
-}
-
-\begingroup
-\catcode`\^^M=13
-\gdef\xsimman@read@options#1#2^^M{%
-  \ifblank{#2}{}{\xsimman@read@options@#2}%
-  #1%
-  \XSIMfilewritestart*{\jobname.tmp}%
-}%
-\endgroup
-
-\def\xsimman@read@options@[#1]{\pgfqkeys{/cnltx}{#1}}
+  }
 
 \RenewDocumentEnvironment{sourcecode}{}
   {%
@@ -983,6 +969,7 @@
 \xsim@add@version{0.16a} {2020-01-16}
 \xsim@add@version{0.17}  {2020-02-21}
 \xsim@add@version{0.18}  {2020-02-23}
+\xsim@add@version{0.19}  {2020-03-16}
 
 % ----------------------------------------------------------------------------
 \newrobustcmd*\xsimauxfileinfo{%
diff --git a/doc/xsim.history b/doc/xsim.history
index cb9fd8e..ca36c27 100644
--- a/doc/xsim.history
+++ b/doc/xsim.history
@@ -148,7 +148,8 @@ HISTORY:
                    - change \printexercise and \printsolution to accept a
                      clist of either ids or IDs (or mixed)
                    - new templates in the `layouts' style
- v0.19  2020/03/11 - improve performance
+ v0.19  2020/03/16 - improve performance
                    - only set \ExerciseID and \ExerciseType locally to ensure
                      correct values when exercises (of different types) are
                      nested
+		   - new load-time option `debug' - only for development
diff --git a/doc/xsim_manual.pdf b/doc/xsim_manual.pdf
index 03d1892..f3d4e64 100644
Binary files a/doc/xsim_manual.pdf and b/doc/xsim_manual.pdf differ
diff --git a/doc/xsim_manual.tex b/doc/xsim_manual.tex
index e301758..d3adf28 100644
--- a/doc/xsim_manual.tex
+++ b/doc/xsim_manual.tex
@@ -68,6 +68,9 @@ \section{Licence, Requirements and \texorpdfstring{\textsc{readme}}{README}}
   package.  Be careful: if you're in the middle of an important project it
   might be better to wait with the update until you've finished the project.
   Every update might be breaking some things.
+
+  Please also be aware that \xsim\ is actively developed and many things may
+  still change.  However, I will try my best to keep the interface stable.
 \end{bewareofthedog}
 
 \section{Motivation and Background}
@@ -1483,6 +1486,10 @@ \subsubsection{Further Commands for Usage in Template Definitions}
     Can be used to refer to the current exercise type.
   \expandable\command{ExerciseID}
     Can be used to refer to the current exercise id.
+  \expandable\command{ExerciseText}
+    Can be used inside solutions to retrieve the text of the corresponding
+    solution.  This is propably seldom useful as in most use cases the
+    exercise property \property{solution} is the easier alternative.
   \expandable\command{ExerciseCollection}
     Can be used in certain templates to refer to the collection that is
     currently inserted.