def __add_namespaceinfo(self, ni):
        """Internal method to directly add a _NamespaceInfo object to this
        set.  No sanity checks are done (e.g. checking for prefix conflicts),
        so be sure to do it yourself before calling this."""
        self.__ns_uri_map[ni.uri] = ni
        for prefix in ni.prefixes:
            self.__prefix_map[prefix] = ni