Opaque Mem.alloc.
Transparent Mem.free.

Lemma free_access:
forall m1 b lo hi m2, Mem.free m1 b lo hi = Some m2 ->
(forall ofs, lo <= ofs < hi ->
access_at m1 (b,ofs) Cur = Some Freeable /\ access_at m2 (b,ofs) Max = None).