intros p s i s' i' H; induction H.
exists 0; apply stepN_O.
destruct IHstepstar as [n Hn].
exists (S n).
eapply stepN_S.
apply H.
apply Hn.
Qed.

Lemma stepN_stepstar : forall p n s i s' i',
stepN p n s i s' i' ->
stepstar p s i s' i'.