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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 etree(self):
 """Returns a lxml object of the response's content that can be selected by xpath"""

        if not hasattr(self, '_elements'):
            try:
                parser = lxml.html.HTMLParser(encoding=self.encoding)
                self._elements = lxml.html.fromstring(self.content, parser=parser)
            except LookupError:
                # lxml would raise LookupError when encoding not supported
                # try fromstring without encoding instead.
                # on windows, unicode is not availabe as encoding for lxml
                self._elements = lxml.html.fromstring(self.content)
        if isinstance(self._elements, lxml.etree._ElementTree):
            self._elements = self._elements.getroot()
        return self._elements