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EXAMPLE
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The a(6) = 48 subsets:
{} {1} {1,2} {1,2,3} {1,2,3,4} {1,2,3,4,5} {1,2,3,4,5,6}
{2} {1,3} {1,2,4} {1,2,3,5} {1,2,3,4,6}
{3} {1,4} {1,2,5} {1,2,3,6} {1,2,3,5,6}
{4} {1,5} {1,3,4} {1,2,4,5} {2,3,4,5,6}
{5} {1,6} {1,3,5} {1,3,4,5}
{6} {2,3} {1,4,5} {1,4,5,6}
{2,4} {1,4,6} {2,3,4,5}
{2,5} {1,5,6} {2,3,4,6}
{3,4} {2,3,4} {2,3,5,6}
{3,5} {2,3,5}
{4,5} {2,3,6}
{4,6} {2,4,5}
{5,6} {3,4,5}
{4,5,6}
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