code: public function handle(ServerRequestInterface $request): ResponseInterface
  	{
  		if ($this->stages->valid()) {
  			$stage = $this->stages->current();
  			$this->stages->next();
  
  			return $stage->process($request, $this);
  		}
  
  		return $this->responseFactory->createResponse(404);
  	}
docs: Handle the request
  
  @param ServerRequestInterface $request
  @return ResponseInterface