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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def sort_index(self, ascending=True):
 """Sort the index of the DataFrame. Currently MultiIndex is not supported since Weld is missing multiple-column sort. Note this is an expensive operation (brings all data to Weld). Parameters ascending : bool, optional Returns ------- DataFrame DataFrame sorted according to the index. """

        if isinstance(self.index, MultiIndex):
            raise NotImplementedError('Weld does not yet support sorting on multiple columns')

        return self.sort_values(self.index._gather_names(), ascending)