reserve x, y for set;
reserve D for non empty set;
reserve UN for Universe;

theorem Th1:
  for u1,u2,u3,u4 being Element of UN holds [u1,u2,u3] in UN & [u1,
  u2,u3,u4] in UN
proof
  let u1,u2,u3,u4 be Element of UN;
  [u1,u2,u3] = [[u1,u2],u3] & [u1,u2,u3,u4] = [[u1,u2,u3],u4];
  hence thesis by CLASSES2:58;
end;
