#vulnerable code 
                                @Override
    public void queueVariable(int level, Var var) {

	int i = varToIndex.get(var);

	indexQueue.add(i);

    } 
                            #location 4 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                                @Override
    public void queueVariable(int level, Var var) {

	Integer iVal = varXToIndex.get(var);
	if (iVal == null)
	    iVal = varYToIndex.get(var);
	int i = iVal.intValue();

	indexQueue.add(i);

    }