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_2: forall chunk b' ofs p', valid_access m' chunk b' ofs p' -> valid_access m chunk b' ofs p'.