(************)
simple induction l1. simpl in |- *. intros. inversion H. simpl in |- *. assumption.
intros. inversion H0. simpl in |- *. apply len_cons. apply H. exact H4.
exact H1.
Qed.

(* Specifications: *)
Lemma len_length : forall v : list A, len v (length v).
simple induction v. auto with arith.