class MSEImpurityMeasure implements ImpurityMeasure<MSEImpurityMeasure> {

    @Override public MSEImpurityMeasure subtract(MSEImpurityMeasure b) {
        return new MSEImpurityMeasure(
            leftY - b.leftY,
            leftY2 - b.leftY2,
            leftCnt - b.leftCnt,
            rightY - b.rightY,
            rightY2 - b.rightY2,
            rightCnt - b.rightCnt
        );
    }

    public  MSEImpurityMeasure(double leftY, double leftY2, long leftCnt, double rightY, double rightY2, long rightCnt);

}