reserve x, y for object, I for set,
  A, B, X, Y for ManySortedSet of I;

theorem :: ZFMISC_1:2
  union EmptyMS I = EmptyMS I
proof
  now
    let i be object;
    assume i in I;
    hence (union EmptyMS I).i = union (EmptyMS I.i) by Def2
      .= {} by PBOOLE:5,ZFMISC_1:2
      .= EmptyMS I.i by PBOOLE:5;
  end;
  hence thesis;
end;
