@Override
    public void free() {
        //if (this == SOME || this == ALL || this == NONE) {
        if (this == ALL || this == NONE) {
            // Those can not change.
            return;
        }
        if (agent != null) {
            agent.free();
            agent = null;
        }
    }
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