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