code: def observable_object_keys(instance):
      
      digits_re = re.compile(r"^\d+$")
      for key in instance['objects']:
          if not digits_re.match(key):
              yield JSONError("'%s' is not a good key value. Observable Objects "
                              "should use non-negative integers for their keys."
                              % key, instance['id'], 'observable-object-keys')
docs: Ensure observable-objects keys are non-negative integers.