NULL
[2] Sentinel value - Wikipedia, 2023-11-13T21:03:50.000Z, 2023-11-25T05:00:16.577Z https://en.wikipedia.org/wiki/Sentinel_value
[1] PEP 661 – Sentinel Values | peps.python.org, 2023-11-24T23:08:14.000Z, 2023-11-25T04:59:59.999Z https://peps.python.org/pep-0661/