Definition perm_order''_dec : forall (op op' : option permission),
      {Mem.perm_order'' op op'} + {~ Mem.perm_order'' op op'}.Definition perm_eq_dec: forall (op op' : option permission),
      {op = op'} + {~ op = op'}.

Lemma free_decay: forall m b lo hi m', Mem.free m b lo hi = Some m' -> decay m m'.