Definition compat_pointer_chunks (chunk1 chunk2: memory_chunk) : Prop :=
  match chunk1, chunk2 with
  | (Mint32 | Many32), (Mint32 | Many32) => True
  | (Mint64 | Many64), (Mint64 | Many64) => True
  | _, _ => False
  end.

Lemma valid_access_drop_1: forall chunk b' ofs p', b' <> b \/ ofs + size_chunk chunk <= lo \/ hi <= ofs \/ perm_order p p' -> valid_access m chunk b' ofs p' -> valid_access m' chunk b' ofs p'.