code: def _sb_r2(self, term, r1_prefixes=None):
          
          r1_start = self._sb_r1(term, r1_prefixes)
          return r1_start + self._sb_r1(term[r1_start:])
docs: Return the R2 region, as defined in the Porter2 specification.
  
          Parameters
          ----------
          term : str
              The term to examine
          r1_prefixes : set
              Prefixes to consider
  
          Returns
          -------
          int
              Length of the R1 region