(task 6)
1     (9009 9090 9900)
2     2920
3     192210
4     19222110
5     19323110
6     1923123110
7     1923224110
8     191413323110
9     191433125110
10     19151423125110
11     19251413226110
12     1916151413325110
13     1916251423127110
14     191716151413326110
15     191726151423128110
16     19181716151413327110
17     19182716151423129110
18     29181716151413328110
19     19281716151423228110
20     19281716151413427110
21     19182716152413228110
1000000     -->     max-sequence=     21     nodes=     10926

(task 7 #f)
10000000     -->     max-sequence=     21     nodes=     23432
(task 8 #f)
100000000     -->     max-sequence=     21     nodes=     47359
(task 9 #f)
1000000000     -->     max-sequence=     21     nodes=     97455
(task 10 #f)
10000000000     -->     max-sequence=     21     nodes=     188493