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_split : forall env1 env2 n e, subst_env' n (env1 ++ env2) e = subst_env' n env1 (subst_env' (length env1 + n) env2 e).