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

Lemma env_ext: forall rho1 rho2, env_get rho1 = env_get rho2 -> rho1=rho2.