public function push($key, $value = null)
    {
        if (is_null($value)) {
            $this->items[] = $key;

            return;
        }

        $items = $this->get($key);

        if (is_array($items) || is_null($items)) {
            $items[] = $value;
            $this->set($key, $items);
        }
    }