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

theorem Th21:
  for X being set st union X in U holds X in U
  proof
    let X be set;
    assume union X in U; then
A1: bool union X in U by CLASSES2:59;
    X c= bool union X by ZFMISC_1:82;
    hence thesis by A1,CLASSES4:13;
  end;
