a(4)=42棵树:
{{1}{1}{12}} {{12}{12}} {{1}{123}} {1234}
{{1}{{1}{12}}} {{1}{2}{12}} {{12}{13}} {{1}{234}}
{{1}{{2}{12}}} {{1}{1}{23}} {{12}{34}}
{{2}{{1}{12}}} {{1}{2}{13}} {{13}{24}}
{{1}{3}{12}} {{14}{23}}
{{1}{{1}{23}}} {{2}{134}}
{{1}{{2}{13}}} {{3}{124}}
{{1}{{3}{12}}} {{4}{123}}
{{2}{{1}{13}}} {{1}{2}{34}}
{{3}{{1}{12}}} {{1}{3}{24}}
{{1}{4}{23}}
{{2}{3}{14}}
{{2}{4}{13}}
{{3}{4}{12}}
{{1}{{2}{34}}}
{{1}{{3}{24}}}
{{1}{{4}{23}}}
{{2}{{1}{34}}}
{{2}{{3}{14}}}
{{2}{{4}{13}}}
{{3}{1}{24}}}
{{3}{2}{14}}}
{{3}{{4}{12}}}
{{4}{{1}{23}}}
{{4}{2}{13}}}
{{4}{{3}{12}}}
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