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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def print_sentence(self, sent_index):
 """ 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 """

        tokens = [self.print_token(tok_idx)
                  for tok_idx in self.sentences[sent_index]]
        return ' '.join(tokens)