public function applyDiscount(?Discount $discount = null): ?string 	{ 		if (!$discount) { 			return null; 		}  		return $discount->is_percentage ? MathMoneyOperations::calculate($this->price, round((float) MathMoneyOperations::calculate($this->price, MathMoneyOperations::calculate((string) $discount->amount, '100', '/'), '*'), 2), '-') : MathMoneyOperations::calculate($this->price, (string) $discount->amount, '-'); 	}