final Money totalAmountToPay = getPostingRule(whenRegistered).calculateTotalAmountToPay(this, whenRegistered);
final Money totalAmountToPay = calculateTotalAmountToPay(whenRegistered);
    public Money calculateAmountToPay(DateTime whenRegistered) {	final Money totalAmountToPay = getPostingRule(whenRegistered).calculateTotalAmountToPay(this,		whenRegistered);	return totalAmountToPay.isPositive() ? totalAmountToPay.subtract(calculatePayedAmount())		: Money.ZERO;    }