code: func (d *SerialDispatcher) AppendHandlers(handlers ...Handler) {
  	d.mutex.Lock()
  	defer d.mutex.Unlock()
  
  Loop:
  	for _, newHandler := range handlers {
  		for _, existingHandler := range d.handlers {
  			if newHandler == existingHandler {
  				continue Loop
  			}
  		}
  		d.handlers = append(d.handlers, newHandler)
  	}
  }
docs: // AppendHandlers implements `Dispatcher.AppendHandlers`