code: def _id_to_subword(self, subword_id):
      
      if subword_id < 0 or subword_id >= (self.vocab_size - 1):
        raise ValueError("Received id %d which is invalid. Ids must be within "
                         "[0, %d)." % (subword_id + 1, self.vocab_size))
  
      if 0 <= subword_id < len(self._subwords):
        # Subword
        return self._subwords[subword_id]
      else:
        # Byte
        offset = len(self._subwords)
        subword_id -= offset
        bytestr = bytes(bytearray([subword_id]))
        return bytestr
docs: Converts a subword integer ID to a subword string.