docs: def compute_discounts(self, precision=None):
          '''
          Returns the total amount of discounts for this line with a specific
          number of decimals.
          @param precision:int number of decimal places
          @return: Decimal
          '''
          gross = self.compute_gross(precision)
          return min(gross,
                     sum([d.compute(gross, precision) for d in self.__discounts]))