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_App : forall env e1 e2, subst_env env (App e1 e2) = App (subst_env env e1) (subst_env env e2).