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例子
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a(2)=12棵树:
{{1,2},{1,3},{2,4}}
{{1,2},{1,3},{3,4}}
{{1,2},{1,4},{2,3}}
{{1,2},{1,4},{3,4}}
{{1,2},{2,3},{3,4}}
{{1,2},{2,4},{3,4}}
{{1,3},{1,4},{2,3}}
{{1,3},{1,4},{2,4}}
{{1,3},{2,3},{2,4}}
{{1,3},{2,4},{3,4}}
{{1,4},{2,3},{2,4}}
{{1,4},{2,3},{3,4}}
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