code: func (ev *changePrev) GetPrevValue(prevVal proto.Message) (exists bool, err error) {
  	if ev.prev != nil {
  		return true, ev.prev.GetValue(prevVal)
  	}
  	return false, nil
  }
docs: // GetPrevValue returns previous value associated with a change. For description of parameter and output
  // values, see the comment in implemented interface datasync.ChangeEvent.