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

theorem Th12:
  for x1,x2,x3,x4 being object st x = [x1,x2,x3,x4] holds
  x1 is Element of U & x2 is Element of U & x3 is Element of U &
  x4 is Element of U
  proof
    let x1,x2,x3,x4 be object;
    assume x = [x1,x2,x3,x4];
    then x = [[x1,x2,x3],x4] by XTUPLE_0:def 8;
    then x4 is Element of U & [x1,x2,x3] is Element of U by CLASSES4:22;
    hence thesis by Th11;
  end;
