theorem
  ord-type {a} = 1
  proof
    a in succ a by ORDINAL1:6; then
A1: {a} c= succ a by ZFMISC_1:31; then
    order_type_of RelIncl {a} = 1 by CARD_5:37;
    hence thesis by A1,ORDINAL3:6;
  end;
