code: def print_sentence(self, sent_index):
          
          tokens = [self.print_token(tok_idx)
                    for tok_idx in self.sentences[sent_index]]
          return ' '.join(tokens)
docs: returns the string representation of a sentence.
  
          :param sent_index: the index of a sentence (from ``self.sentences``)
          :type sent_index: int
          :return: the sentence string
          :rtype: str