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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 _get_sentences_dict(self):
 """ Returns sentence objects :return: order dict of sentences :rtype: collections.OrderedDict """

        if self._sentences_dict is None:
            sentences = [Sentence(element) for element in self._xml.xpath('/root/document/sentences/sentence')]
            self._sentences_dict = OrderedDict([(s.id, s) for s in sentences])
        return self._sentences_dict