Proof star_step.

Lemma star_E0_ind:
forall (P: state -> state -> Prop),
(forall s, P s s) ->
(forall s1 s2 s3, step s1 E0 s2 -> P s2 s3 -> P s1 s3) ->
forall s1 s2, star s1 E0 s2 -> P s1 s2.