private static String toString(int begin, int end) {
        if (begin == 0) {
            return String.format("[%d]**", end);
        } else if (begin > end) {
            return String.format("[%d, %d]*", begin, end);
        }
        return String.format("[%d, %d]", begin, end);
    }
@return A new ZStar1 interval with boundaries [end(zStar1)+1, end(zStar2)]
predicate-search/src/main/java/com/yahoo/search/predicate/index/Interval.java