Definition corestep_plus c m c' m' :=
exists n, corestepN (S n) c m c' m'.
Definition corestep_star c m c' m' :=
exists n, corestepN n c m c' m'.

Lemma corestep_plus_star : forall c1 c2 m1 m2,
corestep_plus c1 m1 c2 m2 -> corestep_star c1 m1 c2 m2.