I find `nextup/`nextdown` useful for when you want to calculate `{f(nextdown(x)), f(x), f(nextup(x))}` and see if some property holds (or to what degree it holds) for both `x` and its surrounding values. It's annoying that one such surrounding value gets skipped.

That said, quite often you need to handle signed zero specially already, and if you don't the property you're testing might sufficiently differ in the interesting way for the smallest-magnitude subnormals.