<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def set_index(self, index):
 """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. """

        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