pairs that I think (a priori) are exchangeable
1 27
1 28
27 28
2 17
2 18
2 19
17 18
17 19
18 19
5 8
5 20 
5 21
5 22
8 20
8 21
8 22
20 21
20 22
21 22
6 12
6 24
12 24
7 25
9 13
9 15
9 16
9 26
13 15
13 16
13 26
15 16
15 26
16 26