code: func lookupHandler(handlerIndex int) (handler, error) {
  	handlerMu.Lock()
  	defer handlerMu.Unlock()
  
  	handle, ok := handlers[handlerIndex]
  	if !ok {
  		return nil, trace.BadParameter("handler with index %v not registered", handlerIndex)
  	}
  
  	return handle, nil
  }
docs: // lookupHandler returns a particular handler from the package level map.