Proof using.
intros a l m H; elim H; simpl in |- *; auto; intuition.
Qed.

(* Compatibility with the append function *)
Theorem permutation_app_comp :
forall l1 l2 l3 l4,
permutation l1 l2 -> permutation l3 l4 -> permutation (l1 ++ l3) (l2 ++ l4).