if (valueNumber == null || knownValue == null ) throw new NullPointerException();
	public void setKnownValue(ValueNumber valueNumber, IsNullValue knownValue) {		assert trackValueNumbers;		knownValueMap.put(valueNumber, knownValue);		if (IsNullValueAnalysis.DEBUG) {			System.out.println("Updated information for " + valueNumber);			System.out.println("                    now " + this);		}	}