code: def tss(self, up=0, down=0):
          
          if not self.is_gene_pred: return None
          tss = self.txEnd if self.strand == '-' else self.txStart
          start, end = tss, tss
          if self.strand == '+':
              start -= up
              end += down
          else:
              start += up
              end -= down
              start, end = end, start
          return max(0, start), max(end, start, 0)
docs: Return a start, end tuple of positions around the transcription-start
          site
  
          Parameters
          ----------
  
          up : int
             if greature than 0, the strand is used to add this many upstream
             bases in the appropriate direction
  
          down : int
             if greature than 0, the strand is used to add this many downstream
             bases into the gene.