Added concept 'isa' other_concept functionality

tests/test_sheerkaDataProvider.py

@@ -836,15 +836,20 @@ def test_i_can_add_a_dictionary_as_a_reference(root):
 ])
 def test_i_can_add_unique(root):
     sdp = SheerkaDataProvider(root)
-    sdp.add_unique(evt_digest, "entry", ObjNoKey(1, "foo"))
-    sdp.add_unique(evt_digest, "entry", ObjNoKey(1, "foo"))
-    sdp.add_unique(evt_digest, "entry", ObjNoKey(2, "bar"))
+    entry, key = sdp.add_unique(evt_digest, "entry", ObjNoKey(1, "foo"))
+    assert (entry, key) == ("entry", None)
+
+    entry, key = sdp.add_unique(evt_digest, "entry", ObjNoKey(1, "foo"))
+    assert (entry, key) == (None, None)
+
     entry, key = sdp.add_unique(evt_digest, "entry", ObjNoKey(2, "bar"))
+    assert (entry, key) == ("entry", None)
+
+    entry, key = sdp.add_unique(evt_digest, "entry", ObjNoKey(2, "bar"))
+    assert (entry, key) == (None, None)
 
     state = sdp.load_state(sdp.get_snapshot())
     assert state.data == {"entry": {ObjNoKey(1, "foo"), ObjNoKey(2, "bar")}}
-    assert entry == "entry"
-    assert key is None
 
 
 @pytest.mark.parametrize("root", [