code: func (m *Memberlist) invokeNackHandler(nack nackResp) {
  	m.ackLock.Lock()
  	ah, ok := m.ackHandlers[nack.SeqNo]
  	m.ackLock.Unlock()
  	if !ok || ah.nackFn == nil {
  		return
  	}
  	ah.nackFn()
  }
docs: // Invokes nack handler if any is associated.