code: def sort_index(self, ascending=True):
          
          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)
docs: 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.