Comments.

ppp_libmodule/simplification.py

@@ -6,19 +6,25 @@ from ppp_datamodel.nodes.list_operators import *
 __all__ = ['simplify']
 
 def simplify_union(tree):
+    # Trivial cases
     if len(tree.list) == 0:
         return List([])
     elif len(tree.list) == 1:
         return tree.list[0]
+
+    # Partition the items in lists / non-lists
     list_ = [x for x in tree.list if x]
     lists = (x.list for x in list_ if isinstance(x, List))
     not_lists = [x for x in list_ if not isinstance(x, List)]
+
+    # Make union of lists
     lists = list(set(itertools.chain(*lists)))
-    if not_lists:
+
+    if not_lists: # If there are non-lists (eg. triples)
         all_ = not_lists
         all_.append(List(lists))
         return Union(all_)
-    else:
+    else: # If there are only lists
         return List(lists)