code: func (lb *LogBroker) markDone(sub *subscription, nodeID string, err error) {
  	lb.mu.Lock()
  	defer lb.mu.Unlock()
  
  	// remove the subscription from the node's subscription list, if it exists
  	if _, ok := lb.subscriptionsByNode[nodeID]; ok {
  		delete(lb.subscriptionsByNode[nodeID], sub)
  	}
  
  	// mark the sub as done
  	sub.Done(nodeID, err)
  }
docs: // markDone wraps (*Subscription).Done() so that the removal of the sub from
  // the node's subscription list is possible