unfold eqwordset in |- *.
intros.
elim (H w); clear H; intros; auto.
Qed.

(* Specifications: *)
Lemma eqwordset_trans :
forall l1 l2 l3 : wordset,
eqwordset l1 l2 -> eqwordset l2 l3 -> eqwordset l1 l3.