public function addItem(HasTax $item, int $amount = 1) : HasTax
{
$calculation = clone $this;
$calculation->basePrice += $item->basePrice() * $amount;
$calculation->taxPrice += $item->taxPrice() * $amount;
return $calculation;
}
public static function fromItem(HasTax $item) : Calculation
{
return new Calculation($item->basePrice(), $item->taxPrice());
}