public static BitMap not(BitMap a) {
        if (a == ALL) {
            return NONE;
        } else if (a == NONE) {
            return ALL;
            //} else if (a == SOME) {
            //    return SOME;
        }
        a.agent.flip(0, a.agent.size());
        return a;
    }
NOT operator. a could be changed and returned.
indexr-common/src/main/java/io/indexr/util/BitMap.java