Section FIX.
Variables A B: Type.
Variable R: A -> A -> Prop.
Hypothesis Rwf: well_founded R.
Variable F: forall (x: A), (forall (y: A), R y x -> B) -> B.
Definition Fix (x: A) : B := Wf.Fix Rwf (fun (x: A) => B) F x.

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