Boundary check, nothing else

A function you trust in the middle and not at the edges.

Check this function at its boundaries only: empty input, one element, maximum size, zero, negative, null. For each, say what happens and whether that is correct or merely what the code happens to do. Skip style, skip performance, skip anything that is not an edge case.

Function:

Reviewing a change. Passes you run over a diff before it merges: correctness, security, concurrency, migrations.