1_1 1_2, 1_3 1_4, 2_4, 3_4 1_5 1_6, 2_6
2_1 2_2
3_1 3_2 3_3 3_5, 4_5 3_6
4_1 4_2 4_3, 4_4 4_6 4_7
5_1, 6_1 5_2, 5_3, 6_2, 6_3 5_4, 6_4 5_5, 6_5 5_6, 6_6 5_7, 6_7