Not initialiazed variables now returns BuiltinConcepts.NOT_INITIALIZED

src/core/builtin_concepts.py

@@ -61,6 +61,7 @@ class BuiltinConcepts(Enum):
     EXPLANATION = "explanation"
     PRECEDENCE = "precedence"  # use to set priority among concepts when parsing
     ASSOCIATIVITY = "associativity"  # use to set priority among concepts when parsing
+    NOT_INITIALIZED = "not initialized"
 
     NODE = "node"
     GENERIC_NODE = "generic node"
@@ -104,7 +105,8 @@ BuiltinErrors = [str(e) for e in {
     BuiltinConcepts.CONCEPT_ALREADY_IN_SET,
     BuiltinConcepts.NOT_A_SET,
     BuiltinConcepts.WHERE_CLAUSE_FAILED,
-    BuiltinConcepts.CHICKEN_AND_EGG
+    BuiltinConcepts.CHICKEN_AND_EGG,
+    BuiltinConcepts.NOT_INITIALIZED
 }]
 
 """

src/core/concept.py

@@ -391,7 +391,8 @@ class Concept:
         :return:
         """
         if prop_name not in self.values:
-            return None
+            from core.builtin_concepts import BuiltinConcepts
+            return BuiltinConcepts.NOT_INITIALIZED
         return self.values[prop_name].value
 
     def variables(self):

src/core/sheerka/Sheerka.py

@@ -657,7 +657,7 @@ class Sheerka(Concept):
         if not isinstance(obj, Concept):
             return obj
 
-        if obj.body is None:
+        if obj.body is BuiltinConcepts.NOT_INITIALIZED:
             return obj
 
         if reduce_simple_list and (isinstance(obj.body, list) or isinstance(obj.body, set)) and len(obj.body) == 1: