code: protected function resolveStack()
  	{
  		if ($this->stack !== null) {
  			return $this->stack;
  		}
  
  		$this->stack = new StackBuilder;
  
  		foreach ($this->middlewares as $middleware) {
  			call_user_func_array([$this->stack, 'push'], (array) $middleware);
  		}
  
  		return $this->stack;
  	}
docs: Resolve the stack builder.
  
  @return \Stack\Builder