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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 createIntSubset(self, name, ExternalID, SystemID):
 """Create the internal subset of a document """

        ret = libxml2mod.xmlCreateIntSubset(self._o, name, ExternalID, SystemID)
        if ret is None:raise treeError('xmlCreateIntSubset() failed')
        __tmp = xmlDtd(_obj=ret)
        return __tmp