theorem Th10:
  card 2Set Seg n = n choose 2
proof
  {Y where Y is Subset of Seg n: card Y = 2} = 2Set Seg n by SGRAPH1:def 2;
  hence card 2Set Seg n = card (Seg n) choose 2 by Th9
    .= n choose 2 by FINSEQ_1:57;
end;
