reserve A,A1,A2,B,C,D for Ordinal,
  X,Y for set,
  x,y,a,b,c for object,
  L,L1,L2,L3 for Sequence,
  f for Function;

theorem Th18:
  sup A = A
proof
  On A = A & for B st On A c= B holds A c= B by Th8;
  hence thesis by Def3;
end;
