public function rememberManyForever(array $keys, Closure $callback)
    {
        $values = $this->getMany($keys);

        $items = Arr::where($values, function ($key, $value) {
            return is_null($value);
        });

        if (! empty($items)) {
            $items = array_combine(array_keys($items), $callback(array_keys($items)));

            $this->foreverMany($items);

            $values = array_replace($values, $items);
        }

        return $values;
    }