code: protected function resolve($index)
      {
          $callback = null;
  
          $stack = $this->stack;
  
          if (isset($this->stack[$index])) {
              $item = $stack[$index];
  
              $next = $this->resolve($index + 1);
  
              $callback = function ($request) use ($item, $next) {
                  return $item->process($request, $next);
              };
          }
  
          return new Delegate($callback);
      }
docs: Resolves the whole stack through its index.
  
  @param  integer $index
  @return \Rougin\Slytherin\Middleware\HandlerInterface