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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 sentence(self):
 """ The sentence related to this mention :getter: returns the sentence this mention relates to :type: corenlp_xml.document.Sentence """

        if self._sentence is None:
            sentences = self._element.xpath('sentence/text()')
            if len(sentences) > 0:
                self._sentence = self._coref.document.get_sentence_by_id(int(sentences[0]))
        return self._sentence