Definition MR (x y: T): Prop := R (m x) (m y).

Lemma rect (Q: forall x, P x -> Type) (inv: forall (x: A) (H: forall (y: A), R y x -> Q y (Wf.Fix_sub A R Rwf P f y)) (a: Acc R x), Q x (f (fun y: {y: A | R y x} => Wf.Fix_sub A R Rwf P f (proj1_sig y)))) : forall x, Q _ (Wf.Fix_sub A R Rwf P f x).