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EXAMPLE
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The a(4) = 56 weakly crossing edge-sets:
{12,13} {12,13,14} {12,13,14,23} {12,13,14,23,24} {12,13,14,23,24,34}
{12,14} {12,13,23} {12,13,14,24} {12,13,14,23,34}
{12,23} {12,13,24} {12,13,14,34} {12,13,14,24,34}
{12,24} {12,13,34} {12,13,23,24} {12,13,23,24,34}
{12,34} {12,14,23} {12,13,23,34} {12,14,23,24,34}
{13,14} {12,14,24} {12,13,24,34} {13,14,23,24,34}
{13,23} {12,14,34} {12,14,23,24}
{13,24} {12,23,24} {12,14,23,34}
{13,34} {12,23,34} {12,14,24,34}
{14,24} {12,24,34} {12,23,24,34}
{14,34} {13,14,23} {13,14,23,24}
{23,24} {13,14,24} {13,14,23,34}
{23,34} {13,14,34} {13,14,24,34}
{24,34} {13,23,24} {13,23,24,34}
{13,23,34} {14,23,24,34}
{13,24,34}
{14,23,24}
{14,23,34}
{14,24,34}
{23,24,34}
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