private static Box initialize() {
        Box[] nodes = new Box[7];
        nodes[1] = new Box(1);
        int[] s = {1, 4, 7};
        for (int i = 0; i < 3; ++i) {
            nodes[2] = new Box(21 + i);
            nodes[1].add(nodes[2]);
            int lev = 3;
            for (int j = 0; j < 4; ++j) {
                nodes[lev - 1].add(new Product(lev * 10 + s[i]));
                nodes[lev] = new Box(lev * 10 + s[i] + 1);
                nodes[lev - 1].add(nodes[lev]);
                nodes[lev - 1].add(new Product(lev * 10 + s[i] + 2));
                lev++;
            }
        }
        return nodes[1];
    }