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 rho n v m, nth_error rho n = Some v -> subst_env' m rho (Var (n + m)) = val_to_exp v.