intros.
cut (x1 = x2 :>Elt /\ w1 = w2 :>Word); [ intuition | auto ].
Qed.

(* Specifications: *)
Lemma cons_cons_inv2 :
forall (x1 x2 : Elt) (w1 w2 : Word), cons x1 w1 = cons x2 w2 -> w1 = w2.