Ltac show_stopped :=
  let m' := fresh "m'" in
  let e' := fresh "e'" in
  let H := fresh "H" in
  right; intros [m' [e' H]]; inv H.

Lemma subst_env_Deref: forall rho e, subst_env rho (Deref e) = Deref (subst_env rho e).