code: func RegisterSaver(saver Saver) (err error) {
  	notOnce := true
  	once.Do(func() {
  		generator.Lock()
  		generator.Saver = saver
  		generator.Unlock()
  		err = generator.init()
  		notOnce = false
  		return
  	})
  	if notOnce {
  		panic("uuid: Register* methods cannot be called more than once.")
  	}
  	return
  }
docs: // RegisterSaver register's a uuid.Saver implementation to the default package
  // uuid.Generator. If you wish to save the generator state, this function must
  // be run before any calls to V1 or V2 UUIDs. uuid.RegisterSaver cannot be run
  // in conjunction with uuid.Init. You may implement the uuid.Saver interface
  // or use the provided uuid.Saver's from the uuid/savers package.