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
 }]
 
 """