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

theorem Th18:
  for x being object holds x in U iff {x} in U
  proof
    let x be object;
    thus x in U implies {x} in U by CLASSES2:57;
    assume {x} in U;
    then x in {x} in U by TARSKI:def 1;
    hence x in U by CLASSES4:def 1;
  end;
