<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def setContextDoc(self, doc):
 """Set the doc of an xpathContext """

        if doc is None: doc__o = None
        else: doc__o = doc._o
        libxml2mod.xmlXPathSetContextDoc(self._o, doc__o)