Create the internal subset of a document

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