code: func RegisterGenerator(config *GeneratorConfig) (err error) {
  	notOnce := true
  	once.Do(func() {
  		generator, err = NewGenerator(config)
  		notOnce = false
  		return
  	})
  	if notOnce {
  		panic("uuid: Register* methods cannot be called more than once.")
  	}
  	return
  }
docs: // RegisterGenerator will set the package generator with the given configuration
  // Like uuid.Init this can only be called once. Any subsequent calls will have no
  // effect. If you call this you do not need to call uuid.Init.