code: def modified_created(instance):
      
      if 'modified' in instance and 'created' in instance and \
              instance['modified'] < instance['created']:
          msg = "'modified' (%s) must be later or equal to 'created' (%s)"
          return JSONError(msg % (instance['modified'], instance['created']),
                           instance['id'])
docs: `modified` property must be later or equal to `created` property