Added ModifyConcept function, and fixed 'isa' not working

src/core/sheerka/Sheerka.py

@@ -34,6 +34,7 @@ class Sheerka(Concept):
 
     CONCEPTS_ENTRY = "All_Concepts"  # to store all the concepts
     CONCEPTS_BY_ID_ENTRY = "Concepts_By_ID"
+    CONCEPTS_BY_HASH_ENTRY = "Concepts_By_Hash" # store hash of concepts definitions (not values)
     CONCEPTS_DEFINITIONS_ENTRY = "Concepts_Definitions"  # to store definitions (bnf) of concepts
     BUILTIN_CONCEPTS_KEYS = "Builtins_Concepts"  # sequential key for builtin concepts
     USER_CONCEPTS_KEYS = "User_Concepts"  # sequential key for user defined concepts
@@ -482,6 +483,7 @@ class Sheerka(Concept):
         # otherwise, create another instance
         concept = self.builtin_cache[key]() if key in self.builtin_cache else Concept()
         concept.update_from(template)
+        concept.freeze_definition_hash()
 
         if len(kwargs) == 0:
             return concept