Record memstep_preserve (P:mem -> mem -> Prop) :=
{
preserve_trans: forall m1 m2 m3, P m1 m2 -> P m2 m3 -> P m1 m3;
preserve_mem: forall m m', mem_step m m' -> P m m'
}.

Lemma storebytes_unch_loc_unwritable b ofs: forall l m m' (L: Mem.storebytes m b ofs l = Some m'),
Mem.unchanged_on (loc_not_writable m) m m'.