intros x y dans_x_u dans_y_u.
elimtype (dans x X \/ dans x V1).
intro dans_x.
replace (injproducg V1 x) with x.
elimtype (dans y X \/ dans y V1).
intros dans_y.
replace (injproducg V1 y) with y.
auto.
apply sym_equal; auto.
intros dans_y.
replace (injproducg V1 y) with (injgauche y).
unfold injgauche in |- *.
elim (Mots_X x dans_x).
intros x0 egal_x egal2_x.
absurd (word x0 = couple y zero).
discriminate.
rewrite egal_x; assumption.
apply sym_equal; auto.
auto.
apply sym_equal; auto.
intro dans_x.
replace (injproducg V1 x) with (injgauche x).
elimtype (dans y X \/ dans y V1).
intros dans_y.
replace (injproducg V1 y) with y.
unfold injgauche in |- *.
elim (Mots_X y dans_y).
intros x0 egal_y egal2_y.
absurd (word x0 = couple x zero).
discriminate.
rewrite egal2_y; assumption.
apply sym_equal; auto.
intro.
replace (injproducg V1 y) with (injgauche y).
unfold injgauche in |- *.
intros.
apply couple_couple_inv1 with zero zero; assumption.
apply sym_equal; auto.
auto.
apply sym_equal; auto.
auto.
Qed.
Hint Resolve is_mono_u_X_V1_injproducg_V1.

Lemma is_mono_u_X_V2_injproducd_V2 : is_mono (union X V2) (injproducd V2).
unfold is_mono in |- *.