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.

Lemma valid_access_dec: forall m chunk b ofs p, {valid_access m chunk b ofs p} + {~ valid_access m chunk b ofs p}.