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 Th8:
  On A = A
proof
  for x being object holds x in A iff x in A & x is Ordinal;
  hence thesis by ORDINAL1:def 9;
end;
