destruct rho.
unfold finite_idfun, finMap in *.
generalize f; intros [l ?].
exists l; simpl in *.
intros; unfold compose in *.
apply e; auto.
Qed.

Lemma finite_idfun_empty: finite_idfun (fun _ => None).
Definition empty_env : env := mk_env _ finite_idfun_empty.