static <A> AutoCloseable tempAdd(Cl<A> l, A a) {
  if (l == null || l.contains(a)) null;
  l.add(a);
  ret new tempAdd_undo(l, a);
}

srecord noeq tempAdd_undo<A>(Cl<A> l, A a) is AutoCloseable {
  close { l.remove(a); }
}

static AutoCloseable tempAdd(Container a, Component b) {
  if (a == null || b == null) null;
  swing {
    if (b.getParent() == a) null;
    a.add(b);
    ret -> swing {
      if (b.getParent() == a)
        removeFromParent(b);
    };
  }
}

Began life as a copy of #1016563