code: func (io IndexOptions) String() string {
  	mopt := map[string]interface{}{}
  	if io.keysSet {
  		mopt["keys"] = io.keys
  	}
  	if io.trackExistenceSet {
  		mopt["trackExistence"] = io.trackExistence
  	}
  	return fmt.Sprintf(`{"options":%s}`, encodeMap(mopt))
  }
docs: // String serializes this index to a JSON string.