private function items(): array
    {
        $items = [];

        foreach ($this->samples as $sample) {
            foreach ($sample as $item) {
                if (!in_array($item, $items, true)) {
                    $items[] = $item;
                }
            }
        }

        return array_map(function ($entry) {
            return [$entry];
        }, $items);
    }