Definition Fix (x: A) : B := Wf.Fix Rwf (fun (x: A) => B) F x.

(* Specifications: *)
Theorem unroll_Fix: forall x, Fix x = F (fun (y: A) (P: R y x) => Fix y).
Theorem unroll_Fix: forall x, Fix x = F (fun (y: A) (P: R y x) => Fix y).