docs: def extra(self):
          """
          Return the extra parameters that this instance. Extra meaning those
          that are not listed in the c_params specification.
          
          :return: The key,value pairs for keys that are not in the c_params
              specification,
          """
          return dict([(key, val) for key, val in
                       self._dict.items() if key not in self.c_param])