def append_rows(self, indexes, values):
        if len(values) != len(indexes):
            raise ValueError('length of values is not equal to length of indexes')
        combined_index = self._index + indexes
        if len(set(combined_index)) != len(combined_index):
            raise IndexError('duplicate indexes in Series')
        self._index.extend(indexes)
        self._data.extend(values)