Remark read_as_zero_unchanged:
  forall (P: block -> Z -> Prop) m b ofs len m',
  read_as_zero m b ofs len ->
  Mem.unchanged_on P m m' ->
  (forall i, ofs <= i < ofs + len -> P b i) ->
  read_as_zero m' b ofs len.

Lemma store_zeros_read_as_zero: forall m b p n m', store_zeros m b p n = Some m' -> read_as_zero m' b p n.