Proof with auto.
intros.
destruct x0...
simpl.
f_equal...
f_equal...
Qed.

(* Specifications: *)
Lemma unfold: forall l, qs l = Fix_sub (list T) (MR lt (fun l0 : list T => length l0)) qs_obligation_3 (fun _ : list T => list T) body l.
Section rect.