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_compat: forall m chunk1 chunk2 b ofs p, size_chunk chunk1 = size_chunk chunk2 -> align_chunk chunk2 <= align_chunk chunk1 -> valid_access m chunk1 b ofs p-> valid_access m chunk2 b ofs p.