proposition: Lemma eqset_diff_if_right : forall T deq (l l1 l2 : list T), @eqset T l1 l2 -> @eqset T (diff deq l1 l) (diff deq l2 l).
proof: Proof. unfold eqset; introv eqs; introv. repeat (rw in_diff). rw eqs; sp. Qed.
imports: UsefulTypes bin_rels Omega