reserve a,b,c,d for Ordinal;
reserve l for non empty limit_ordinal Ordinal;
reserve u for Element of l;
reserve A for non empty Ordinal;
reserve e for Element of A;
reserve X,Y,x,y,z for set;
reserve n,m for Nat;

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;
