A key may exist in an ingest table, a stable table or both, a tombstone key may exist in the ingest table and the corresponding key may or may not be in the stable table.  In all, there are 5 possibilities.  We should have a test that tests iteration transitioning from and to all of these possibilities.  In developing such a test, I have an example that crashes, attached as test_layered88_fails.py.  It creates "situations" testing such transitions.  While debugging it, I observed that if I run each situation by itself, the test succeeds, but running them altogether gives a failure.

Apart from debugging the failure, the test could be improved, but not only iterating forwards, but also backwards.