code: def __add_namespaceinfo(self, ni):
          
          self.__ns_uri_map[ni.uri] = ni
          for prefix in ni.prefixes:
              self.__prefix_map[prefix] = ni
docs: 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.