Looking at recent discussions about Set (SE-0203 — Rename Sequence.elementsEqual - #51 by adamkemp), it's a pity that we can't just replace that with your datatype - the current situation should make every mathematician cry ;-)
But I guess our "collection with hidden order and without duplicates" is more useful for common applications...
Have you thought about starting with Predicate, and building a set type on top of that? NSPredicate needs an overhaul as well ;-)