code: def _coord(sequence, start, mirna, precursor, iso):
      
      dif = abs(mirna[0] - start)
      if start < mirna[0]:
          iso.t5 = sequence[:dif].upper()
      elif start > mirna[0]:
          iso.t5 = precursor[mirna[0] - 1:mirna[0] - 1 + dif].lower()
      elif start == mirna[0]:
          iso.t5 = "NA"
      if dif > 4:
          logger.debug("start > 3 %s %s %s %s %s" % (start, len(sequence), dif, mirna, iso.format()))
          return None
  
      end = start + (len(sequence) - len(iso.add)) - 1
      dif = abs(mirna[1] - end)
      if iso.add:
          sequence = sequence[:-len(iso.add)]
      # if dif > 3:
      #    return None
      if end > mirna[1]:
          iso.t3 = sequence[-dif:].upper()
      elif end < mirna[1]:
          iso.t3 = precursor[mirna[1] - dif:mirna[1]].lower()
      elif end == mirna[1]:
          iso.t3 = "NA"
      if dif > 4:
          logger.debug("end > 3 %s %s %s %s %s" % (len(sequence), end, dif, mirna, iso.format()))
          return None
      logger.debug("%s %s %s %s %s %s" % (start, len(sequence), end, dif, mirna, iso.format()))
      return True
docs: Define t5 and t3 isomirs