1  2  3  4  5  6  7  8  9 10
  +-------------------------------
1 |  0  1  3  6 10 15 21 28 36 45
2 |  1  4  9 14 21
3 |  3  9 16
4 |  6 10    ??

4 8 7 6 0  ->  0 1 2 3 4
9 3 2 1 5      5 6 7 8 9