proposition: Lemma is_epi_f_over_image : is_epi E (map f E) f.
proof: Proof. split. auto. intros. cut (exists y : Elt, dans y E /\ x = f y :>Elt). intro temp; elim temp; clear temp. intro. intuition. prolog [ ex_intro2 ] 4. auto. Qed.
imports: Ensf Words more_words need