Definition l_egal (l1 l2 : wordset) : Prop :=
  l_inclus l1 l2 /\ l_inclus l2 l1.

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).