proposition: Lemma null_app : forall T, forall l1 l2 : list T, null (l1 ++ l2) <=> null l1 # null l2.
proof: Proof. induction l1; simpl; sp; split; sp; try (apply null_nil); try(apply null_cons in H); sp; try(apply null_cons in H0); sp. Qed.
imports: UsefulTypes bin_rels Omega