End FIX.
(** Same, with a nonnegative measure instead of a well-founded ordering *)
Section FIXM.
Variables A B: Type.
Variable measure: A -> nat.
Variable F: forall (x: A), (forall (y: A), measure y < measure x -> B) -> B.
Definition Fixm (x: A) : B := Wf.Fix (well_founded_ltof A measure) (fun (x: A) => B) F x.

Theorem unroll_Fixm:
forall x, Fixm x = F (fun (y: A) (P: measure y < measure x) => Fixm y).