case a.
auto.
unfold wef in |- *; simpl in |- *; intros x w H; discriminate H.
Qed.

Lemma wef_cons :
forall (a b : Word) (e : Elt),
cons e a = wef b ->
exists x : Elt,
ex2 (fun w : Word => cons x w = b) (fun w : Word => f x = e /\ wef w = a).