Set the index where MIDDLE starts.

        :param int index: the new index for MIDDLE begin

def middle_begin(self, index):
        """
        Set the index where MIDDLE starts.

        :param int index: the new index for MIDDLE begin
        """
        if (index < 0) or (index > self.all_length):
            raise ValueError(u"The given index is not valid")
        self.__middle_begin = index