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EXAMPLE
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The a(2) = 1 through a(9) = 18 subsets:
{1,2} {1,3} {1,2,4} {1,2,5} {1,2,6} {2,3,7} {3,5,8} {4,6,9}
{2,3} {1,3,4} {1,4,5} {1,3,6} {2,4,7} {4,5,8} {5,6,9}
{2,3,5} {1,4,6} {2,6,7} {1,2,4,8} {1,2,4,9}
{2,4,5} {1,5,6} {3,4,7} {1,2,6,8} {1,2,6,9}
{2,3,6} {4,5,7} {1,3,4,8} {1,2,7,9}
{2,5,6} {4,6,7} {1,3,7,8} {1,3,4,9}
{3,4,6} {1,2,5,7} {1,5,6,8} {1,3,8,9}
{3,5,6} {1,3,6,7} {1,5,7,8} {1,4,8,9}
{2,3,6,8} {1,6,7,9}
{2,4,7,8} {1,6,8,9}
{2,3,5,9}
{2,3,7,9}
{2,4,5,9}
{2,4,8,9}
{2,6,7,9}
{2,6,8,9}
{3,4,7,9}
{3,5,8,9}
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