(************)
Qed.

(* Specifications: *)
Lemma rev_rev : forall l : list A, rev (rev l) = l.
simple induction l; auto with arith. intros; simpl in |- *. rewrite rev_app. rewrite H. auto with arith.
Lemma length_rev : forall l : list A, length (rev l) = length l.
simple induction l; auto with arith. intros. simpl in |- *.