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 corestepN_rdonly n c m c' m' (CS:corestepN M n c m c' m') b (VB:Mem.valid_block m b): readonly m b m'.