svoid keepComponentsByPred(final Container container, final F1<Component, Bool> keep) swing {
  bool changed = false;
  if (container != null) for (int i = 0; i < numberOfChildren(container); i++) {
    Component c = getComponentAtIndex(container, i);
    if (!callF(keep, c)) {
      container.remove(c);
      set changed;
      --i;
    }
  }
  if (changed) revalidate(container);
}