unfold Derivestar in |- *.
prolog [ Rstar_inv ] 6.
(*Intros R u v Der_R. Apply Rstar_inv;Assumption.*)
Qed.
Hint Resolve Derivestar_inv.

Lemma Derivestar_inclus :
forall (R1 R2 : Ensf) (u v : Word),
inclus R1 R2 -> Derivestar R1 u v -> Derivestar R2 u v.