(******************)
intros. rewrite H0. auto with arith.
Qed.
Definition app_assoc_r := app_ass.
(*********************)
Definition app_assoc_l := ass_app.
(*********************)

Lemma len_cons_not_O :
forall (b : A) (v : list A) (l : nat),
len (b :: v) l -> l <> 0.