code: def utr5(self):
          
          if not self.is_coding or len(self.exons) < 2: return (None, None)
          if self.strand == "+":
              s, e = (self.txStart, self.cdsStart)
          else:
              s, e = (self.cdsEnd, self.txEnd)
          if s == e: return (None, None)
          return s, e
docs: return the 5' UTR if appropriate