intros p n s i s' i' H; induction H.
apply stepstar_O.
eapply stepstar_S.
apply H.
apply IHstepN.
Qed.

Lemma step_deterministic : forall p s i s1 s2 i1 i2,
step p s i s1 i1 ->
step p s i s2 i2 ->
s1 = s2 /\ i1 = i2.