Transparent Mem.free.

(* Specifications: *)
Lemma mem_free_obs:
forall f m m' lo hi b1 b2 m2
(Hmem_obs_eq: mem_obs_eq f m m')
(Hf: f b1 = Some b2)
(Hfree: Mem.free m b1 lo hi = Some m2),
exists m2',
Mem.free m' b2 lo hi = Some m2' /\
mem_obs_eq f m2 m2'.