Gets the total of the invoice with a defined decimal precision
        @param precision: int Number of decimal places
        @return: Decimal

def compute_total(self, precision=None):
        '''
        Gets the total of the invoice with a defined decimal precision
        @param precision: int Number of decimal places
        @return: Decimal
        '''
        return quantize((self.compute_gross(precision) +
                        self.compute_taxes(precision) -
                        self.compute_discounts(precision)),
                        places=precision)