Computes the amount of the treatment.
        @param base:float Gross
        @return: Decimal

def compute(self, base, precision=None):
        '''
        Computes the amount of the treatment.
        @param base:float Gross
        @return: Decimal
        '''
        if base <= ZERO:
            return ZERO

        if self.rate_type == RATE_TYPE_FIXED:
            if not self.interval or base >= self.interval.lower:
                return quantize(self.rate, precision)
            return ZERO

        if not self.interval:
            return quantize(base * self.rate / 100, precision)

        if base > self.interval.lower:
            base = min(base, self.interval.upper) - self.interval.lower
            return quantize(base * self.rate / 100, precision)

        return ZERO