Definition FreeEffect m lo hi (sp b:Values.block) (ofs:Z): bool :=
   if valid_block_dec m b
   then eq_block b sp && zle lo ofs && zlt ofs hi
   else false.

Lemma FreeEffect_free: forall m sp lo hi m' (FREE: Mem.free m sp lo hi = Some m'), Mem.unchanged_on (fun b ofs => FreeEffect m lo hi sp b ofs = false) m m'.