Section Fix_rects.
Variable A: Type.
Variable P: A -> Type.
Variable R : A -> A -> Prop.
Variable Rwf : well_founded R.
Variable f: forall (x : A), (forall y: { y: A | R y x }, P (proj1_sig y)) -> P x.

Lemma F_unfold x r:
Fix_F_sub A R P f x r =
f (fun y => Fix_F_sub A R P f (proj1_sig y) (Acc_inv r (proj2_sig y))).