docs: def is_copyright_model(instance, attribute, value):
      """Must include at least a ``rightsOf`` key, but not a ``source``
      key (``rightsOf`` indicates that the Right contains full rights to
      an existing Manifestation or Work; i.e. is a Copyright).
      """
  
      rights_of = value.get('rightsOf')
      if not isinstance(rights_of, str):
          instance_name = instance.__class__.__name__
          raise ModelDataError(("'rightsOf' must be given as a string in "
                                "the '{attr}' parameter of a '{cls}'. Given "
                                "'{value}'").format(attr=attribute.name,
                                                    cls=instance_name,
                                                    value=rights_of))