unfold Derivestar in |- *.
prolog [ Rstar_R ] 8.
(*Intros. Apply Rstar_R with v;Assumption.*)
Qed.

Lemma Derivestar_inv :
forall (R : Ensf) (u v : Word),
Derivestar R u v ->
u = v \/ (exists2 w : Word, Derive R u w & Derivestar R w v).