Notation "s '▷^' n t" := (evaluatesin n s t) (at level 40, n at level 5).
Definition stepplus s t := (exists s', s ≻ s' /\ s' ≻* t).
Notation "s '≻^+' t" := (stepplus s t) (at level 40).

Lemma unique_step_index s m t n : s ▷^m t -> s ▷^n t -> m = n.