generalize V1; clearbody C Gim; clear V1.
simple induction V1; auto.
intros a V1' Hyp Id_X_a_V1_f.
unfold map in |- *.
simpl in |- *.
apply add_add.
auto.
apply Hyp.
apply Id_inclus with (union X (add a V1')).
red in |- *; intros x dans_x_union_X_V1.
cut (dans x X \/ dans x V1').
intros [HX| HV1]; auto.
auto.
assumption.
Qed.

Lemma Id_image_R : Id (union X V1) f -> isGram X V1 R1 S1 -> Rim = R1 :>Ensf.
intros Id_X_V1_f.