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_subst_neq : forall e n m v1 v2, n <> m -> subst n v1 (subst m v2 e) = subst m v2 (subst n v1 e).