Set Implicit Arguments.
Set Strict Implicit.
Fixpoint guard A (R : A -> A -> Prop) (n : nat) (wfR : well_founded R)
{struct n}: well_founded R :=
match n with
| 0 => wfR
| S n => fun x => Acc_intro x (fun y _ => guard n (guard n wfR) y)
end.
Section setoid_fix.
Variables (A : Type) (R : A -> A -> Prop) (Rwf : well_founded R).
Variables (P : A -> Type)
(F : forall x : A, (forall y : A, R y x -> P y) -> P x).
Variable r : forall x : A, P x -> P x -> Prop.
Hypothesis Hstep : forall (x : A) (f g : forall y : A, R y x -> P y),
(forall (y : A) (p : R y x), r (f y p) (g y p)) ->
r (@F x f) (@F x g).

Theorem Fix_equiv
: forall x : A,
r (Fix Rwf P F x) (@F x (fun (y : A) (_ : R y x) => Fix Rwf P F y)).