I am reminded of this annoying footgun:
If the map did not have this key present, None is returned.
If the set did not have this value present, true is returned.
that one has bitten me so many times
fwiw I personally consider the bool return type on HashSet::insert to be a mistake, we should've made it an enum (with methods to check for each variant, perhaps)
I don't think I've ever gotten the true/false to match with what I actually wanted in code