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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 doc(self):
 """Get the document tree from a parser context. """

        ret = libxml2mod.xmlParserGetDoc(self._o)
        if ret is None:raise parserError('xmlParserGetDoc() failed')
        __tmp = xmlDoc(_obj=ret)
        return __tmp