public boolean isEmpty() {
        if (this == ALL) {
            return false;
        } else if (this == NONE) {
            return true;
        }
        int next = agent.nextSetBit(0);
        return next == -1 || next >= bits;
    }
A wrapper of the underlying bitmap.

We guarantee that the content of {@link #ALL} or {@link #NONE} will never be changed.
As those static instances are shared around.
indexr-common/src/main/java/io/indexr/util/BitMap.java