if (nodes == null) return false;
        public boolean isDontCare(int var) {            if (levels == null)                throw new BDDException();             int level = factory.var2Level(var);            int i = Arrays.binarySearch(levels, level);            if (i < 0)                throw new BDDException("var "+var+" not in iteration set");             return nodes[i] == null;        }