proposition: Lemma combine_odd_even_is_id:forall (s:Str), (combine (even s) (odd s))(=)s.
proof: Proof. cofix. intros [x [y xs]]. rewrite even_spelled. rewrite combine_spelled_2. rewrite odd_spelled. constructor. reflexivity. rewrite combine_spelled. constructor. reflexivity. simpl. apply combine_odd_even_is_id. Defined.
imports: List Streams Omega