code: def _fetch_3ss_fasta(fasta, gene_name, exon_num,
                       chrom, strand, start, end):
      
  
      if strand == '-':
          ss_seq = fasta.fetch(reference=chrom,
                               start=end-1,
                               end=end+3)
          ss_seq = utils.rev_comp(ss_seq)
      elif strand == '+':
          ss_seq = fasta.fetch(reference=chrom,
                               start=start-3,
                               end=start+1)
      ss_fasta = '>{0};exon{1};3SS\n{2}\n'.format(gene_name,
                                                  exon_num,
                                                  ss_seq.upper())
      return ss_fasta
docs: Retreives the 3' SS sequence flanking the specified exon.
  
      Returns a string in fasta format with the first line containing
      a ">" and the second line contains the two base pairs of 3' SS.
  
      Parameters
      ----------
      fasta : pysam.Fastafile
          fasta object from pysam
      gene_name : str
          gene name used for fasta seq id
      exon_num : int
          the `exon_num` exon, used for seq id
      chrom : str
          chromsome
      strand : str
          strand, {'+', '-'}
      start : int
          0-based start position
      end : int
          0-based end position
  
      Returns
      -------
      ss_fasta : str
          string in fasta format with first line being seq id