Definition range_perm (m: mem) (b: block) (lo hi: Z) (k: perm_kind) (p: permission) : Prop :=
  forall ofs, lo <= ofs < hi -> perm m b ofs k p.

Theorem valid_access_implies: forall m chunk b ofs p1 p2, valid_access m chunk b ofs p1 -> perm_order p1 p2 -> valid_access m chunk b ofs p2.