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 closed_subst_env : forall env e x n, closed' (length env + x) e -> closed' (x + n) (subst_env' (x + n) env e).