{1}
{2}
{1,2}
{1,3}
{2,3}
{2,4}
{3,5}
{3,7}
(George Sicherman)
{1,2,3}
{1,2,4}
{1,3,4}
{1,3,5}
{1,3,6}
{1,3,7}
{2,3,4}
{2,3,5}
{2,3,6}
{2,3,7}
{2,4,5}
{2,4,6}
