code: def make_slash_number(self):
          
          if self.partitioning == 'by codon position' and self.codon_positions == '1st-2nd':
              return '\\2'
          elif self.partitioning in ['by codon position', '1st-2nd, 3rd'] and self.codon_positions in ['ALL', None]:
              return '\\3'
          else:
              return ''
docs: Charset lines have \2 or \3 depending on type of partitioning and codon
          positions requested for our dataset.
  
          :return: