proposition: Lemma diff_dup2 : forall T eq, forall l1 l2 l3 : list T, forall x, LIn x l1 -> diff eq (l1 ++ l2) l3 = diff eq (l1 ++ x :: l2) l3.
proof: Proof. intros; apply diff_dup. apply in_app_iff; left; auto. Qed.
imports: UsefulTypes bin_rels Omega