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1 2 3
A
B
A1 A2
B1 B2
2 3 4
A
B
A2 A3 A4
B2 B3 B4
1 2 3
C
D
B1 B2
C1 C2
D1 D2
2 3 4
C
D
B2 B3 B4
C2 C3 C4
D2 D3 D4