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

theorem Th11:
  for x1,x2,x3 being object st x = [x1,x2,x3] holds
  x1 is Element of U & x2 is Element of U & x3 is Element of U
  proof
    let x1,x2,x3 be object;
    assume x = [x1,x2,x3];
    then x = [[x1,x2],x3] by XTUPLE_0:def 4;
    then x3 is Element of U & [x1,x2] is Element of U by CLASSES4:22;
    hence thesis by CLASSES4:22;
  end;
