code: def set_index(self, index):
          
          if index is None:
              pass
          elif isinstance(index, str):
              index = SortedIndex(self[index], copy=False)
          elif isinstance(index, (tuple, list)) and len(index) == 2:
              index = SortedMultiIndex(self[index[0]], self[index[1]],
                                       copy=False)
          else:
              raise ValueError('invalid index argument, expected string or '
                               'pair of strings, found %s' % repr(index))
          self.index = index
docs: Set or reset the index.
  
          Parameters
          ----------
          index : string or pair of strings, optional
              Names of columns to use for positional index, e.g., 'POS' if table
              contains a 'POS' column and records from a single
              chromosome/contig, or ('CHROM', 'POS') if table contains records
              from multiple chromosomes/contigs.