public static BitMap and(BitMap a, BitMap b) {
        if (a == NONE || b == NONE) {
            return NONE;
        } else if (a == ALL) {
            return b;
        } else if (b == ALL) {
            return a;
            //} else if (a == SOME || b == SOME) {
            //    return SOME;
        }
        a.agent.and(b.agent);
        return a;
    }
AND operator. a could be changed and returned.
indexr-common/src/main/java/io/indexr/util/BitMap.java