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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 response_doc(self):
 """The XML document received from the service."""

        try:
            return self.__dict__['response_doc']
        except KeyError:
            self.__dict__['response_doc'] = ElementTree.fromstring(
                self.response_xml
            )
            return self.__dict__['response_doc']