private function sortMiddleware()
    {
        $this->sorted = [];

        if (!$this->interposeFn) {
            foreach ($this->steps as $step) {
                foreach ($step as $fn) {
                    $this->sorted[] = $fn[0];
                }
            }
            return;
        }

        $ifn = $this->interposeFn;
        // Interpose the interposeFn into the handler stack.
        foreach ($this->steps as $stepName => $step) {
            foreach ($step as $fn) {
                $this->sorted[] = $ifn($stepName, $fn[1]);
                $this->sorted[] = $fn[0];
            }
        }
    }