diff --git a/phylogeny.py b/phylogeny.py
index 1da364e..8226e82 100644
--- a/phylogeny.py
+++ b/phylogeny.py
@@ -265,17 +265,19 @@ def get_descendants(bb):
             assert len(b.clades)==2
             self.B2 = get_descendants(b.clades[0])
             self.B3 = get_descendants(b.clades[1])
-            self.B1 = set(i for i in range(n) if i not in self.B2 and i not in self.B3)
+            self.B1 = set(i for i in range(n) if i not in (self.B2 | self.B3))
 
         def __str__(self):
             return f'{self.a}->{self.b} {self.B1} {self.B2} {self.B3}'
 
     class PreparedTree:
-        def __init__(self,T,n,index):
+        def __init__(self,T,n,index,forward=True):
             self.n = n
             self.T = read(StringIO(T), 'newick')
             self.number_nodes()
-            self.edges,self.expanded_nodes = self.create_edges()
+            self.internal_nodes = self.create_internal_nodes(forward=forward)
+            self.expanded_nodes = self.create_expanded_nodes(forward=forward)
+            self.edges          = self.create_edges(forward=forward)
 
         def number_nodes(self):
             m = 0
@@ -286,20 +288,40 @@ def number_nodes(self):
                     clade.name = n+m
                     m+=1
 
-        def create_edges(self):
+        def create_internal_nodes(self,forward=True):
+            product = [clade for clade in self.T.find_clades(order='postorder',terminal=False)]
+            return product if forward else product[::-1]
+
+        def create_expanded_nodes(self,forward=True):
+            product = {clade.name : self.get_leaves(clade) for clade in self.T.find_clades(order='postorder',terminal=False)}
+            if forward:
+                for clade in self.internal_nodes:
+                    for child in clade.clades:
+                        if child.name in product:
+                            product[clade.name] |= product[child.name]
+            else:
+                parent = {}
+                for clade in self.internal_nodes:
+                    for child in clade.clades:
+                        if child.name>= self.n:
+                            parent[child.name] = clade.name
+                for clade in self.internal_nodes:
+                    if clade.name in parent:
+                        product[clade.name] |= product[parent[clade.name]]
+
+            return product
+
+        def get_leaves(self,clade):
+            return set(child.name for child in clade.clades if child.name<self.n)
+
+        def create_edges(self,forward=True):
             edges          = []
-            expanded_nodes = {}
             for internal_node in self.T.find_clades(order='postorder',terminal=False):
-                expanded_nodes[internal_node.name] = []
                 for child in internal_node.clades:
-                    if child.name<self.n:
-                        expanded_nodes[internal_node.name].append(child.name)
-                    else:
-                        edges.append(Edge(internal_node,child, expanded_nodes, self.n ))
-                        for descendant in expanded_nodes[child.name]:
-                            expanded_nodes[internal_node.name].append(descendant)
+                    if child.name>=self.n:
+                        edges.append(Edge(internal_node,child, self.expanded_nodes, self.n ))
 
-            return edges,expanded_nodes
+            return edges
 
     def get_count_for_pair(e1,e2):
         F1 = e1.B1
@@ -308,15 +330,13 @@ def get_count_for_pair(e1,e2):
         G1 = e2.B1
         G2 = e2.B2
         G3 = e2.B3
-        return comb(len(F1.intersection(G1)),2,exact=True) *           \
-               (    len(F2.intersection(G2))*len(F3.intersection(G3))+ \
-                    len(F2.intersection(G3))*len(F3.intersection(G2)))
+        return comb(len(F1&G1),2,exact=True) *  (len(F2&G2)*len(F3&G3)+ len(F2&G3)*len(F3&G2))
 
     n     = len(species)
     index = {species[i]:i for i in range(n)}
     tree1 = PreparedTree(T1,n,index)
-    tree2 = PreparedTree(T2,n,index)
-    return 2*comb(n,2) - 2*sum(get_count_for_pair (e1,e2) for e1 in tree1.edges for  e2 in tree2.edges)
+    tree2 = PreparedTree(T2,n,index,forward=False)
+    return 2*comb(n,4) - 2*sum(get_count_for_pair (e1,e2) for e1 in tree1.edges for  e2 in tree2.edges)
 
 
 
@@ -826,11 +846,12 @@ def is_trivial(self):
         def is_singleton(self):
             return len(self.taxa)==1
 
-        # newick
-        #
-        # Convert to string in Newick format
-
         def newick(self):
+            '''
+            newick
+
+            Convert to string in Newick format
+            '''
             def conv(taxon):
                 if type(taxon)==int:
                     return species[taxon]
@@ -841,14 +862,17 @@ def conv(taxon):
             else:
                 return '(' + ','.join(conv(taxon) for taxon in self.taxa) +')'
 
-        # split
-        #
-        # Split clade in two using character: list of taxa is replaced by two clades
-        #
-        # Returns True if clade has been split into two non-trivial clades
-        #         False if at least one clade would be trivial--in which case clade is unchanged
-        #
+
         def split(self,character):
+            '''
+            split
+
+            Split clade in two using character: list of taxa is replaced by two clades
+
+            Returns True if clade has been split into two non-trivial clades
+                    False if at least one clade would be trivial--in which case clade is unchanged
+
+            '''
             left  = []
             right = []
             for i in self.taxa:
@@ -863,10 +887,13 @@ def split(self,character):
             self.taxa = [leftTaxon,rightTaxon]
             return True
 
-        # splitAll
-        #
-        # Split clade using character table
+
         def splitAll(self,characters,depth=0):
+            '''
+            splitAll
+
+            Split clade using character table
+            '''
             if depth<len(characters):
                 if self.split(characters[depth]):
                     for taxon in self.taxa:
@@ -874,10 +901,10 @@ def splitAll(self,characters,depth=0):
                 else:
                     self.splitAll(characters,depth+1)
 
-
-    # Calculate entropy of a single character
-
     def get_entropy(freq):
+        '''
+        Calculate entropy of a single character
+        '''
         if freq==0 or freq==n: return 0
         p1 = freq/n
         p2 = 1-p1
@@ -1031,7 +1058,7 @@ def conflicts_with(c1, c2):
         return True
 
     n         = len(table)
-    Conflicts = [0 for _ in range(n)]   # Count number of times each row conflicts with another
+    Conflicts = np.zeros(n)   # Count number of times each row conflicts with another
     for i in range(n):
         for j in range(i+1,n):
             if conflicts_with(table[i],table[j]):
@@ -1040,8 +1067,6 @@ def conflicts_with(c1, c2):
 
     return [table[row] for row in range(n) if row!=np.argmax(Conflicts)]
 
-
-
 def cntq(n,newick):
     '''CNTQ Counting Quartets'''
     def create_adj(tree):