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_subst_env_lt : forall env n m v e, n < m -> subst n v (subst_env' m env e) = subst_env' m env (subst n v e).