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_two: forall c m c' m' c'' m'', corestep Sem ge c m c' m' -> corestep Sem ge c' m' c'' m'' -> corestep_plus c m c'' m''.