Not initialiazed variables now returns BuiltinConcepts.NOT_INITIALIZED

tests/non_reg/test_sheerka_non_reg.py

@@ -64,7 +64,7 @@ class TestSheerkaNonRegMemory(TestUsingMemoryBasedSheerka):
 
         # sanity check
         evaluated = sheerka.evaluate_concept(self.get_context(eval_body=True), return_value)
-        assert evaluated == simplec("un", simplec("one", None))
+        assert evaluated == simplec("un", simplec("one", BuiltinConcepts.NOT_INITIALIZED))
 
     def test_i_can_recognize_concept_with_no_body(self):
         sheerka = self.get_sheerka()
@@ -734,7 +734,7 @@ as:
         assert len(res) == 1
         assert res[0].status
         assert sheerka.isinstance(res[0].body, "foobar")
-        assert res[0].body.body is None
+        assert res[0].body.body == BuiltinConcepts.NOT_INITIALIZED
 
         res = sheerka.evaluate_user_input("eval foo bar")
         assert len(res) == 1