public static CombineIndicator minus(Indicator<Num> indicatorLeft, Indicator<Num> indicatorRight) {
        return new CombineIndicator(indicatorLeft, indicatorRight, Num::minus);
    }
@Test
    public void testDifferenceIndicator() {

        Function<Number, Num> numFunction = DecimalNum::valueOf;

        BarSeries series = new BaseBarSeries();
        FixedIndicator<Num> mockIndicator = new FixedIndicator<Num>(series, numFunction.apply(-2.0),
                numFunction.apply(0.00), numFunction.apply(1.00), numFunction.apply(2.53), numFunction.apply(5.87),
                numFunction.apply(6.00), numFunction.apply(10.0));
        ConstantIndicator<Num> constantIndicator = new ConstantIndicator<Num>(series, numFunction.apply(6));
        CombineIndicator differenceIndicator = CombineIndicator.minus(constantIndicator, mockIndicator);

        assertNumEquals("8", differenceIndicator.getValue(0));
        assertNumEquals("6", differenceIndicator.getValue(1));
        assertNumEquals("5", differenceIndicator.getValue(2));
        assertNumEquals("3.47", differenceIndicator.getValue(3));
        assertNumEquals("0.13", differenceIndicator.getValue(4));
        assertNumEquals("0", differenceIndicator.getValue(5));
        assertNumEquals("-4", differenceIndicator.getValue(6));
    }