def next(self):
        """Advance the cursor."""
        if self.__empty:
            raise StopIteration
        if len(self.__data) or self._refresh():
            if self.__manipulate:
                _db = self.__collection.database
                return _db._fix_outgoing(self.__data.popleft(),
                                         self.__collection)
            else:
                return self.__data.popleft()
        else:
            raise StopIteration
Advance the cursor.