Returns the total amount of payments made to this invoice.
        @param precision:int Number of decimal places
        @return: Decimal

def compute_payments(self, precision=None):
        '''
        Returns the total amount of payments made to this invoice.
        @param precision:int Number of decimal places
        @return: Decimal
        '''
        return quantize(sum([payment.amount for payment in self.__payments]),
                        precision)