code: def object_marking_circular_refs(instance):
      
      if instance['type'] != 'marking-definition':
          return
  
      if 'object_marking_refs' in instance:
          for ref in instance['object_marking_refs']:
              if ref == instance['id']:
                  yield JSONError("`object_marking_refs` cannot contain any "
                                  "references to this marking definition object"
                                  " (no circular references).", instance['id'])
docs: Ensure that marking definitions do not contain circular references (ie.
      they do not reference themselves in the `object_marking_refs` property).