Now this is a toy example because usually you never do division this way, but in mature code in commercial applications this is usually what it looks like. It's a sliver of business logic that in itself seems trivial, and then handlers of edge case upon edge case upon edge case, mirroring an even larger set of unit tests.

One reason for this is that you typically lack a type system that allows 'making illegal states unrepresentable' to some extent, or possibly lack a team that can leverage the available type system to that effect due to organisational pressure, insufficient experience or whatever.