private void stackChange(int change)
    {
        if (change <= 0) {
            itsStackDepth += change;
        } else {
            int newDepth = itsStackDepth + change;
            if (newDepth > itsData.itsMaxStack) {
                itsData.itsMaxStack = newDepth;
            }
            itsStackDepth = newDepth;
        }
    }