Not initialiazed variables now returns BuiltinConcepts.NOT_INITIALIZED

tests/core/test_SheerkaEvaluateConcept.py

@@ -9,7 +9,7 @@ from tests.TestUsingMemoryBasedSheerka import TestUsingMemoryBasedSheerka
 class TestSheerkaEvaluateConcept(TestUsingMemoryBasedSheerka):
 
     @pytest.mark.parametrize("body, expected", [
-        (None, None),
+        (None, BuiltinConcepts.NOT_INITIALIZED),
         ("", ""),
         ("1", 1),
         ("1+1", 2),
@@ -132,7 +132,7 @@ class TestSheerkaEvaluateConcept(TestUsingMemoryBasedSheerka):
 
         evaluated = sheerka.evaluate_concept(context, concept)
 
-        assert evaluated == CB("foo", CB("a", None))
+        assert evaluated == CB("foo", CB("a", BuiltinConcepts.NOT_INITIALIZED))
         assert evaluated.metadata.is_evaluated
         assert evaluated.body.metadata.is_evaluated
 
@@ -176,9 +176,9 @@ class TestSheerkaEvaluateConcept(TestUsingMemoryBasedSheerka):
         evaluated = sheerka.evaluate_concept(context, concept_d)
 
         assert evaluated.key == concept_d.key
-        expected = CB("c", CB("b", CB("a", None)))
+        expected = CB("c", CB("b", CB("a", BuiltinConcepts.NOT_INITIALIZED)))
         assert evaluated.body == expected
-        assert sheerka.objvalue(evaluated) == CB("a", None)
+        assert sheerka.objvalue(evaluated) == CB("a", BuiltinConcepts.NOT_INITIALIZED)
         assert evaluated.metadata.is_evaluated
 
     def test_i_can_evaluate_concept_when_variables_reference_others_concepts(self):