proposition: Lemma teq_two_one : teq two zwei -> teq one eins.
proof: Proof. intros; cofix CIH. apply teq_fold. rewrite (tunf_eq one), (tunf_eq eins); simpl. constructor; auto. apply teq_fold. rewrite (tunf_eq two); simpl. constructor; auto. Defined.
imports: Setoid Program Paco.paco