reserve U for Universe;
reserve x for Element of U;

theorem Th13:
  for x1,x2,x3,x4,x5 being Element of U holds [x1,x2,x3,x4,x5] is Element of U
  proof
    let x1,x2,x3,x4,x5 be Element of U;
    [x1,x2,x3,x4] is Element of U & [x1,x2,x3,x4,x5] = [[x1,x2,x3,x4],x5]
      by GRCAT_1:1;
    hence thesis by CLASSES2:58;
  end;
