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