code: func SetInstance(kvdb Kvdb) error {
  	if instance == nil {
  		lock.Lock()
  		defer lock.Unlock()
  		if instance == nil {
  			instance = kvdb
  			return nil
  		}
  	}
  	return fmt.Errorf("Kvdb instance is already set to %q", instance.String())
  }
docs: // SetInstance sets the singleton instance.