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 c m c' m' -> corestep  Sem c' m' c'' m'' ->
corestep_plus c m c'' m''.