intros p s i s1 s2 i1 i2 H1 H2.
inversion H1; subst; inversion H2; subst;
intuition congruence.
Qed.

(* Specifications: *)
Lemma step_confluent : forall p n s s1 s2 s' i i1 i2 i',
step p s i s1 i1 ->
step p s i s2 i2 ->
stepN p n s1 i1 s' i' ->
stepN p n s2 i2 s' i'.