if (IsNullValueAnalysisFeatures.TRACK_KNOWN_VALUES) {
if (trackValueNumbers) {
	public void copyFrom(Frame<IsNullValue> other) {		super.copyFrom(other);		if (IsNullValueAnalysisFeatures.TRACK_KNOWN_VALUES) {			knownValueMap = new HashMap<ValueNumber, IsNullValue>(((IsNullValueFrame)other).knownValueMap);		}	}