I could have used such an aggregate in the past, so +1. This is maybe getting into nit-picking, but perhaps it should be documented as returning an "arbitrary" value instead of a "non-deterministic" one? Technically the value is deterministic: there's a concrete algorithm specifying how it's selected. However, the algorithm is reserved as an implementation detail, since the function is designed for cases in which the caller should not care which value is returned.
Thanks, Maciek
