rewrite (Fix_F_eq x0 r); rewrite (Fix_F_eq x0 s); intros.
apply F_ext; auto.
Qed.
Definition Fix (x : A) := Fix_F x (Rwf x).

Lemma fix_eq : forall x : A, Fix x = F x (fun (y : A) (p : R y x) => Fix y).
intro; unfold Fix in |- *.