Transparent Mem.alloc.

Lemma permission_at_free_2 :
forall m m' (lo hi : Z) b b' ofs'
(Hfree: Mem.free m b lo hi = Some m')
(Hb: b <> b'),
forall k : perm_kind,
permission_at m b' ofs' k = permission_at m' b' ofs' k.