code: def _fetch_seq(self):
          
          exons = []
          three_prime_ss = []
          five_prime_ss = []
          num_exons = self.bed.get_num_exons()
          for i in range(num_exons):
              # add exon sequence
              tmp_id = '{0};exon{1}'.format(self.bed.gene_name, i)
              tmp_exon = self.fasta.fetch(reference=tmp_id)
              exons.append(tmp_exon)
  
              # add splice site sequence
              tmp_id_3ss = '{0};3SS'.format(tmp_id)
              tmp_id_5ss = '{0};5SS'.format(tmp_id)
              if num_exons == 1:
                  pass
              elif i == 0:
                  tmp_5ss = self.fasta.fetch(tmp_id_5ss)
                  five_prime_ss.append(tmp_5ss)
              elif i == (num_exons - 1):
                  tmp_3ss = self.fasta.fetch(tmp_id_3ss)
                  three_prime_ss.append(tmp_3ss)
              else:
                  tmp_3ss = self.fasta.fetch(tmp_id_3ss)
                  tmp_5ss = self.fasta.fetch(tmp_id_5ss)
                  three_prime_ss.append(tmp_3ss)
                  five_prime_ss.append(tmp_5ss)
          return exons, five_prime_ss, three_prime_ss
docs: Fetches gene sequence from PySAM fasta object.
  
          Returns
          -------
          exons : list of str
              list of exon nucleotide sequences
          five_prime_ss : list of str
              list of 5' splice site sequences
          three_prime_ss : list of str
              list of 3' splice site sequences