theorem Th21:
  0 is Element of UN & 1 is Element of UN & 2 is Element of UN
proof
  thus 0 is Element of UN by CLASSES2:56;
  {} is Element of UN & 1 = succ 0 by CLASSES2:56;
  hence
A1: 1 is Element of UN by Th20;
  2 = succ 1;
  hence thesis by A1,Th20;
end;
