0 -> x1
x1 -> x2
x2 -> x1
x2 -> x3
x3 -> x2
x1 -> x4
x4 -> x1
x4 -> x5
x5 -> x4
x1 -> x6
x6 -> x1
x6 -> x7
x7 -> x6
x6 -> x8
x8 -> x6
0 -> x9
x1 -> x10
x10 -> x1
x10 -> x11
x11 -> x10
x10 -> x12
x12 -> x10
x12 -> x13
x13 -> x12
x13 -> x14
x14 -> x13
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