intro n. pattern n.
apply (well_founded_ind lt_wf).
intros.
destruct x. inversion_clear H0. inversion_clear H1...
inversion_clear H0.
inversion_clear H1.
rewrite (@elemsRuniq _ _ CR (nats 0 (S x))) with l l0...
intros.
apply H with x0...
destruct (In_nats_inv _ _ _ H1)...
Qed.
(* .. which is even computable: *)

Lemma CRimpl: forall n (a: Acc lt n), sig (CR n).
Proof with eauto.