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_Prim : forall env f e, subst_env env (Prim f e) = Prim f (subst_env env e).