Fix bad boolean condition

Author: brandonwillard

kanren/facts.py

@@ -67,7 +67,7 @@ def goal(substitution):
 
             for fact in facts:
                 unified = unify(fact, args2, substitution)
-                if unified != False:
+                if unified is not False:
                     yield merge(unified, substitution)
 
         return goal