Clean code and add docstest

Author: mikulatomas

concepts/algorithms/covering_edges.py

@@ -8,7 +8,63 @@
 
 def covering_edges(concept_list, context):
     """Yield mapping edge as ``((extent, intent), (lower_extent, lower_intent))``
-    pairs (concept and it's lower neighbor) from ``context`` and ``concept_list``"""
+    pairs (concept and it's lower neighbor) from ``context`` and ``concept_list``
+    
+    Example:
+        >>> from concepts import make_context
+        >>> from concepts.concepts import ConceptList, Concept
+
+        >>> context = make_context('''
+        ...  |0|1|2|3|4|5|
+        ... A|X|X|X| | | |
+        ... B|X| |X|X|X|X|
+        ... C|X|X| | |X| |
+        ... D| |X|X| | | |''')
+
+        >>> concepts = [('ABCD', ''),
+        ...             ('ABC', '0'),
+        ...             ('AC', '01'),
+        ...             ('A', '012'),
+        ...             ('', '012345'),
+        ...             ('C', '014'),
+        ...             ('AB', '02'),
+        ...             ('B', '02345'),
+        ...             ('BC', '04'),
+        ...             ('ACD', '1'),
+        ...             ('AD', '12'),
+        ...             ('ABD', '2')]
+
+        >>> concept_list = ConceptList.frompairs(
+        ...     map(lambda c: (context._Objects.frommembers(c[0]),
+        ...                    context._Properties.frommembers(c[1])),
+        ...         concepts))
+
+        >>> edges = covering_edges(concept_list, context)
+        
+        >>> [(''.join(concept[0].members()), # doctest: +NORMALIZE_WHITESPACE
+        ...   ''.join(lower[0].members()))
+        ...  for concept, lower in edges]
+        [('ABCD', 'ABC'),
+         ('ABCD', 'ACD'),
+         ('ABCD', 'ABD'),
+         ('ABC', 'AC'),
+         ('ABC', 'AB'),
+         ('ABC', 'BC'),
+         ('AC', 'A'),
+         ('AC', 'C'),
+         ('A', ''),
+         ('C', ''),
+         ('AB', 'A'),
+         ('AB', 'B'),
+         ('B', ''),
+         ('BC', 'C'),
+         ('BC', 'B'),
+         ('ACD', 'AC'),
+         ('ACD', 'AD'),
+         ('AD', 'A'),
+         ('ABD', 'AB'),
+         ('ABD', 'AD')]
+    """
     Objects = context._Objects
     Properties = context._Properties
 
@@ -17,9 +73,9 @@ def covering_edges(concept_list, context):
     for extent, intent in concept_index.items():
         candidate_counter = dict.fromkeys(concept_index, 0)
 
-        property_candidates = Properties.supremum - intent
+        property_candidates = Properties.fromint(Properties.supremum & ~intent)
 
-        for atom in Properties.fromint(property_candidates).atoms():
+        for atom in property_candidates.atoms():
             extent_candidate = Objects.fromint(extent & atom.prime())
             intent_candidate = concept_index[extent_candidate]
             candidate_counter[extent_candidate] += 1