public function incrementOrDecrementStockCount(string $productNumber, int $amount) : array
    {
        Assert::that($productNumber)->minLength(1, 'The length of $productNumber has to be > 0');

        $oldStockCount = $this->getStockCount($productNumber);
        $newStockCount = $oldStockCount + $amount;

        $this->setStockCount($productNumber, $newStockCount);

        return [
            'oldStockCount' => $oldStockCount,
            'newStockCount' => $newStockCount,
        ];
    }