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
  card dom numbering X = card On X
  proof
    dom numbering X = ord-type X &
    ex a st On X c= a by Th18,Def1;
    hence thesis by CARD_5:7;
  end;
