Returns the total discounts of this group.
        @param precision: int Number of decimals
        @return: Decimal

def compute_discounts(self, precision=None):
        '''
        Returns the total discounts of this group.
        @param precision: int Number of decimals
        @return: Decimal
        '''
        return sum([group.compute_discounts(precision) for group
                    in self.__groups])