It seems bad to me, but I don’t see an obvious fix given all the constraints

But maybe that’s just knowing too much and “seems bad” is just the right answer

Thinking about this more, I think it’s bad

Because you could have two functions in different libraries, one which returns a set of integers, and another which accepts a set of numbers, and if a client hooks the two together and the latter function tries set-add, it explodes and there’s no obvious explanation of that to the caller