code: func (s *StanServer) addSubscription(ss *subStore, sub *subState) error {
  	// Store in client
  	if !s.clients.addSub(sub.ClientID, sub) {
  		return fmt.Errorf("can't find clientID: %v", sub.ClientID)
  	}
  	// Store this subscription in subStore
  	if err := ss.Store(sub); err != nil {
  		s.clients.removeSub(sub.ClientID, sub)
  		return err
  	}
  	return nil
  }
docs: // addSubscription adds `sub` to the client and store.