Im not sure about this, we have kind of a mixed terminology right now,  `addnRandom` and `testRatingMatches` arn't really set-theoretic and are best thought of informally, but some of the other ones like `union` are best thought of set-theoretically. Ill make it complement since an easy set-theoretic is available, but we might want to think about the naming in general if we are aiming for consistent.