*/
final class InvalidatableSet<E> extends ForwardingSet<E> {
  private final Supplier<Boolean> validator;
  private final Set<E> delegate;
  private final Supplier<String> errorMessage;

  static <E> InvalidatableSet<E> of(
      Set<E> delegate, Supplier<Boolean> validator, Supplier<String> errorMessage) {
    return new InvalidatableSet<>(

 */
final class InvalidatableSet<E> extends ForwardingSet<E> {
  private final Supplier<Boolean> validator;
  private final Set<E> delegate;
  private final Supplier<String> errorMessage;

  static <E> InvalidatableSet<E> of(
      Set<E> delegate, Supplier<Boolean> validator, Supplier<String> errorMessage) {
    return new InvalidatableSet<>(

  Set<Integer> wrappedSet;
  Set<Integer> copyOfWrappedSet;
  InvalidatableSet<Integer> setToTest;

  @Before
  public void createSets() {
    wrappedSet = new HashSet<>();
    wrappedSet.add(1);
    wrappedSet.add(2);
    wrappedSet.add(3);

    copyOfWrappedSet = ImmutableSet.copyOf(wrappedSet);
    setToTest =
        InvalidatableSet.of(wrappedSet, () -> wrappedSet.contains(1), () -> 1 + "is not present");
  }

  }

  protected final <T> Set<T> nodeInvalidatableSet(Set<T> set, N node) {
    return InvalidatableSet.of(
        set, () -> nodes().contains(node), () -> String.format(NODE_REMOVED_FROM_GRAPH, node));
  }

  protected final <T> Set<T> nodePairInvalidatableSet(Set<T> set, N nodeU, N nodeV) {
    return InvalidatableSet.of(
        set,
        () -> nodes().contains(nodeU) && nodes().contains(nodeV),