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EXAMPLE
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Non-isomorphic representatives of the a(1) = 1 through a(4) = 15 antichains:
{} {} {} {}
{{1}} {{1}} {{1}}
{{1},{2}} {{1,2}} {{1,2}}
{{1},{2}} {{1},{2}}
{{1},{2,3}} {{1,2,3}}
{{1},{2},{3}} {{1},{2,3}}
{{1,2},{1,3}}
{{1},{2},{3}}
{{1},{2,3,4}}
{{1,2},{3,4}}
{{1},{2},{3,4}}
{{1},{2},{3},{4}}
{{2},{1,3},{1,4}}
{{1,2},{1,3},{2,3}}
{{4},{1,2},{1,3},{2,3}}
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