Section Well_founded.
Variable A : Type.
Variable R : A -> A -> Prop.
Hypothesis Rwf : well_founded R.
Variable P : A -> Type.
Variable F_sub : forall x:A, (forall y: { y : A | R y x }, P (proj1_sig y)) -> P x.
Fixpoint Fix_F_sub (x : A) (r : Acc R x) : P x :=
F_sub x (fun y: { y : A | R y x}  => Fix_F_sub (proj1_sig y)
(Acc_inv r (proj2_sig y))).
Definition Fix_sub (x : A) := Fix_F_sub x (Rwf x).
Register Fix_sub as program.wf.fix_sub.
(* Notation Fix_F := (Fix_F_sub P F_sub) (only parsing). (* alias *) *)
(* Definition Fix (x:A) := Fix_F_sub P F_sub x (Rwf x). *)
Hypothesis F_ext :
forall (x:A) (f g:forall y:{y:A | R y x}, P (`y)),
(forall y:{y : A | R y x}, f y = g y) -> F_sub x f = F_sub x g.

Lemma Fix_F_inv : forall (x:A) (r s:Acc R x), Fix_F_sub x r = Fix_F_sub x s.