intro is_iso_f.
split.
auto. (* Hint : dans_inv_f*)
(* Hints Resolve inv1 .*)
intros x dans_x.
exists (f x); [ apply sym_equal; auto | elim is_iso_f ].
intros is_epi_f. elim is_epi_f.
auto.
Qed.
Let wef := Word_ext f.

Lemma application_imp_application_words :
application E F f -> application_words E F wef.