[0]->[]
[0_0]->[]
[0_0_0]->[]
[0_0_0_0]->[]
[0_0_0_1]->[]
[0_0_1]->[]
[0_0_2_1]->[]
[0_1]->[]
[0_2_1]->[]
[0_2_2_1]->[0_0_2_1]
[0_3_2_1]->[0_0_2_1]
[1]->[]
[1_0_0_0]->[]
[1_1]->[]
[1_2_1]->[]
[2_0_0_0]->[0_0_0_0]
[2_1]->[]
[2_2_1]->[0_0_2_1]
[2_2_1]->[]
[3_2_1]->[0_2_1]
[3_2_1]->[]
[]->[]