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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 coreferences(self):
 """ Returns a list of Coreference classes :getter: Returns a list of coreferences :type: list of corenlp_xml.coreference.Coreference """

        if self._coreferences is None:
            coreferences = self._xml.xpath('/root/document/coreference/coreference')
            if len(coreferences) > 0:
                self._coreferences = [Coreference(self, element) for element in coreferences]
        return self._coreferences