docs: protected void doNotifyListenersOfRemovedValues(Set<R> oldItems) {
          List<SetValueChangeListener<R>> listenersCopy = new ArrayList<SetValueChangeListener<R>>(listeners);
          Set<R> unmodifiable = Collections.unmodifiableSet(oldItems);
          for (SetValueChangeListener<R> listener : listenersCopy) {
              listener.valuesRemoved(this, unmodifiable);
          }
      }