intros a b e.
unfold wef in |- *.
case b.
simpl in |- *; intro H; discriminate H.
simpl in |- *; intros x w H.
exists x.
exists w.
trivial.
injection H; auto.
Qed.
End more_about_words.
Hint Resolve wef_cons.

Lemma Append_assoc :
forall a b c : Word, Append a (Append b c) = Append (Append a b) c.