elim X.
auto.
(* Clear X. *)
intros a X' Hyp Id_a_X_V1_f.
unfold map in |- *.
simpl in |- *.
apply add_add.
auto.
apply Hyp.
apply Id_inclus with (union (add a X') V1).
red in |- *; intros x dans_x_union_X_V1.
cut (dans x X' \/ dans x V1).
intros [HX| HV1]; auto.
auto.
assumption.
Qed.

(* Specifications: *)
Lemma Id_image_V : Id (union X V1) f -> Vim = V1 :>Ensf.
unfold Vim, Gim2, Gim, imageGram; simpl in |- *.