0 -> x1
x1 <-> x2
x2 <-> x3
x1 <-> x4
x4 <-> x5
x1 <-> x6
x6 <-> x7
x6 <-> x8
0 -> x9
x1 <-> x10
x10 <-> x11
x10 <-> x12
x12 <-> x13
x13 <-> x14
0 -> x16
x17 -> 0
x20 -> 0
0 -> x21
x22 -> 0
x9 -> 0
x20 -> 0
x8 -> 0
x13 -> 0
x12 -> 0
x3 -> 0
x2 -> 0
x16 -> 0
x11 -> 0
x21 -> 0
x21 -> 0
x21 -> 0
x1 -> 0