public boolean get(int id) {
        if (this == ALL) {
            return true;
        } else if (this == NONE) {
            return false;
        }
        return agent.get(id);
    }
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