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'_Var': forall n m rho, subst_env' (S (n + m)) rho (Var n) = Var n.