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

(* Specifications: *)
Lemma equiv_l_egal_eqwordset : forall a b : wordset, l_egal a b <-> eqwordset a b.
Lemma refl_l_egal : forall l1 : wordset, l_egal l1 l1.
Lemma wef_append : forall u v : Word, wef (Append u v) = Append (wef u) (wef v).
Lemma wef_nil : forall a : Word, wef a = nil -> a = nil.
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).
Lemma Append_assoc : forall a b c : Word, Append a (Append b c) = Append (Append a b) c.