intros x y Derg.
elim Derg.
intros u0 v0 A dans_A_u0_R u v eg_App.
elimtype (u = nil \/ (exists w : Word, (exists x : Elt, u = cons x w))).
intros u_eg.
exists (Append u0 v0).
rewrite u_eg.
trivial.
replace v with (cons A v0).
apply Deriveg1; assumption.
replace v with (Append u v).
auto.
rewrite u_eg.
trivial.
intro temp; elim temp; clear temp.
intros w temp; elim temp; clear temp.
intros x0 u_eg inmon_u.
absurd (dans x0 V).
apply inter_dans with X.
apply isGram2 with R S'.
exact Gram.
apply inmonoid_cons_inv2 with w.
rewrite <- u_eg.
assumption.
replace x0 with A.
apply Regles_inv1 with X R (word u0).
exact Regles_X_V_R.
assumption.
apply cons_cons_inv1 with v0 (Append w v).
change (cons A v0 = Append (cons x0 w) v) in |- *.
rewrite <- u_eg.
auto.
clear eg_App.
generalize u; clear u; simple induction u.
auto.
intros x0 w hyp.
apply or_intror.
exists w.
exists x0.
trivial.
intros u0 v0 x0 dans_x0_X Derg_u_v Hyp u v.
elimtype (u = nil \/ (exists w : Word, (exists x : Elt, u = cons x w))).
intro eg_u.
rewrite eg_u.
intros App_eg inmon_nil.
exists (cons x0 v0).
trivial.
replace v with (cons x0 u0).
apply Deriveg2.
assumption.
assumption.
intro temp; elim temp; clear temp.
intros w temp; elim temp; clear temp.
intros x1 u_eg.
rewrite u_eg.
intros App_cons_eg inmonoid_X_cons_x1_w.
elimtype
(ex2 (fun w1 : Word => Append w w1 = v0) (fun w1 : Word => Deriveg X R v w1)).
intros w0 App_eg Der_v_w0.
exists w0.
change (cons x1 (Append w w0) = cons x0 v0) in |- *.
apply cons_cons.
apply cons_cons_inv1 with (Append w v) u0.
assumption.
assumption.
assumption.
apply Hyp.
apply cons_cons_inv2 with x1 x0.
assumption.
apply inmonoid_cons_inv with x1.
assumption.
generalize u; clear u; simple induction u.
auto.
intros x2 w hyp.
apply or_intror.
exists w.
exists x2.
trivial.
Qed.

Lemma Derivegstar_imp_Der_inmon :
forall x y : Word,
Derivegstar X R x y ->
forall u v : Word,
Append u v = x ->
inmonoid X u ->
ex2 (fun w : Word => Append u w = y) (fun w : Word => Derivegstar X R v w).