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 stepn_trans : forall n m st1 st2 st3, stepn n st1 st2 -> stepn m st2 st3 -> stepn (n+m) st1 st3.