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 step_deterministic: forall m e m1 e1 m2 e2, step (m,e) (m1,e1) -> step (m,e) (m2,e2) -> m1 = m2 /\ e1 = e2.