Added ModifyConcept function, and fixed 'isa' not working

tests/evaluators/test_AddConceptInSetEvaluator.py

@@ -51,18 +51,20 @@ class TestAddConceptInSetEvaluator(TestUsingMemoryBasedSheerka):
         assert res.value.body == "bar"
 
     def test_i_can_add_concept_to_a_set_of_concept(self):
-        context = self.get_context()
-        foo = Concept("foo")
-        context.sheerka.create_new_concept(context, foo)
-
-        bar = Concept("bar")
-        context.sheerka.create_new_concept(context, bar)
-
+        sheerka, context, foo, bar = self.init_concepts("foo", "bar", create_new=True)
         ret_val = get_ret_val("foo", "bar")
+
         res = AddConceptInSetEvaluator().eval(context, ret_val)
+        foo = sheerka.new("foo")  # reload it
 
         assert res.status
         assert context.sheerka.isinstance(res.value, BuiltinConcepts.SUCCESS)
+        assert context.sheerka.isaset(context, bar)
+        assert context.sheerka.isinset(foo, bar)
+        assert context.sheerka.isa(foo, bar)
+
+        foo_from_sheerka = context.sheerka.get("foo")
+        assert foo_from_sheerka.get_prop(BuiltinConcepts.ISA) == [bar]
 
     def test_i_can_add_concept_with_a_body_to_a_set_of_concept(self):
         context = self.get_context()