docs: def get_default_reference(self, method):
          """
          Returns the default reference for a method.
  
          :arg method: name of a method
          :type method: :class:`str`
  
          :return: reference
          :rtype: :class:`Reference <pyxray.descriptor.Reference>` or :class:`str`
          """
          if method not in self._available_methods:
              raise ValueError('Unknown method: {0}'.format(method))
          return self._default_references.get(method)