code: def serialize_text(self):
          '''Returns a serialized form of the Namepace.
          All the elements in the namespace are sorted by
          URI, joined to the associated prefix with a colon and
          separated with spaces.
          :return: bytes
          '''
          if self._uri_to_prefix is None or len(self._uri_to_prefix) == 0:
              return b''
          od = collections.OrderedDict(sorted(self._uri_to_prefix.items()))
          data = []
          for uri in od:
              data.append(uri + ':' + od[uri])
          return ' '.join(data).encode('utf-8')