code: func (t *Set) MarshalJSON() ([]byte, error) {
  	repr := map[string]interface{}{
  		"type": t.typeMarker(),
  	}
  	if t.of != nil {
  		repr["of"] = t.of
  	}
  	return json.Marshal(repr)
  }
docs: // MarshalJSON returns the JSON encoding of t.