(**************)
unfold strip in |- *. intros. rewrite <- rev_eq1.
apply rev_eq. rewrite <- length_rev. rewrite minus_le_O. auto with arith.
rewrite length_rev. exact H.
Qed.

Lemma strip_O : forall v : list A, strip v 0 = v.
intro. unfold strip in |- *. rewrite <- minus_n_O. rewrite <- length_rev.