proposition: Lemma Derive_inv1 : forall (R : Ensf) (u v : Word), Derive R u v -> Derive_inv R u v.
proof: Proof. intros R x y Der_x_y. unfold Derive_inv in |- *. elim Der_x_y; prolog [ ex_intro2 refl_equal or_intror or_introl ] 8. Qed.
imports: Ensf Words more_words need fonctions Relations