unfold well_founded.
cut (forall a: M, forall a0: T, m a0 = a -> Acc MR a0).
intros.
apply (H (m a))...
apply (@well_founded_ind M R wf (fun mm => forall a, m a = mm -> Acc MR a)).
intros.
apply Acc_intro.
intros.
unfold MR in H1.
rewrite H0 in H1.
apply (H (m y))...
Defined.
End measure_wf.
Section Fix_measure_rects.
Variable A: Set.
Variable R: A -> A -> Prop.
Variable Rwf: well_founded R.
Variable P: A -> Type.
Variable f: forall x: A, (forall y: { y: A | R y x }, P (proj1_sig y)) -> P x.

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