reserve U for Universe;
reserve x for Element of U;
reserve U1,U2 for Universe;

theorem
  for x being set holds {x} is U-petit
  proof
    let x be set;
    reconsider u = {} as Element of U by CLASSES2:56;
    card {x} = 1 & card {u} = 1 by CARD_1:30;
    hence thesis by CARD_1:5;
  end;
