proposition: Lemma diff_app_l : forall T eq, forall l1 l2 l3 : list T, diff eq l1 (diff eq l2 l3) = diff eq (l1 ++ l2) l3.
proof: Proof. induction l1; simpl; sp. repeat (rewrite diff_remove). rewrite IHl1; auto. Qed.
imports: UsefulTypes bin_rels Omega