def pop(self, key, *args):
        try:
            o = self.data.pop(key)()
        except KeyError:
            o = None
        if o is None:
            if args:
                return args[0]
            raise KeyError, key
        else:
            return o

    def setdefault(self, key, default=None):
        try:
            o = self.data[key]()
        except KeyError:
            o = None
        if o is None:
            self.data[key] = KeyedRef(default, self._remove, key)
            return default
        else:
            return o