* @author George van den Driessche
 */
@GwtCompatible
public interface Feature<T> {
  /** Returns the set of features that are implied by this feature. */
  Set<Feature<? super T>> getImpliedFeatures();

  private final Set<Feature<? super Set>> implied;

  SetFeature(Feature<? super Set>... implied) {
    this.implied = Helpers.copyToSet(implied);
  }

  @Override
  public Set<Feature<? super Set>> getImpliedFeatures() {
    return implied;
  }

  @Retention(RetentionPolicy.RUNTIME)
  @Inherited
  @TesterAnnotation
  public @interface Require {
    public abstract SetFeature[] value() default {};

  /**
   * Indicates that elements from {@code Multiset.entrySet()} update to reflect changes in the
   * backing multiset.
   */
  ENTRIES_ARE_VIEWS;

  @Override
  public Set<Feature<? super Multiset>> getImpliedFeatures() {
    return Collections.emptySet();
  }

  @Retention(RetentionPolicy.RUNTIME)
  @Inherited
  @TesterAnnotation
  public @interface Require {
    public abstract MultisetFeature[] value() default {};

 * @author George van den Driessche
 */
@GwtCompatible
public interface Feature<T> {
  /** Returns the set of features that are implied by this feature. */
  Set<Feature<? super T>> getImpliedFeatures();

  private final Set<Feature<? super Set>> implied;

  SetFeature(Feature<? super Set>... implied) {
    this.implied = Helpers.copyToSet(implied);
  }

  @Override
  public Set<Feature<? super Set>> getImpliedFeatures() {
    return implied;
  }

  @Retention(RetentionPolicy.RUNTIME)
  @Inherited
  @TesterAnnotation
  public @interface Require {
    public abstract SetFeature[] value() default {};

  private final Set<Feature<? super List>> implied;

  ListFeature(Feature<? super List>... implied) {
    this.implied = Helpers.copyToSet(implied);
  }

  @Override
  public Set<Feature<? super List>> getImpliedFeatures() {
    return implied;
  }

  @Retention(RetentionPolicy.RUNTIME)
  @Inherited
  @TesterAnnotation
  public @interface Require {
    ListFeature[] value() default {};

  MultimapFeature(Feature<? super Multimap>... implied) {
    this.implied = Helpers.copyToSet(implied);
  }

  @Override
  public Set<Feature<? super Multimap>> getImpliedFeatures() {
    return implied;
  }

  @Retention(RetentionPolicy.RUNTIME)
  @Inherited
  @TesterAnnotation
  public @interface Require {
    public abstract MultimapFeature[] value() default {};

  MultimapFeature(Feature<? super Multimap>... implied) {
    this.implied = Helpers.copyToSet(implied);
  }

  @Override
  public Set<Feature<? super Multimap>> getImpliedFeatures() {
    return implied;
  }

  @Retention(RetentionPolicy.RUNTIME)
  @Inherited
  @TesterAnnotation
  public @interface Require {
    public abstract MultimapFeature[] value() default {};

    void misbehave();
  }

  enum ExampleBaseFeature implements Feature<ExampleBaseInterface> {
    BASE_FEATURE_1,
    BASE_FEATURE_2;

    @Override
    public Set<Feature<? super ExampleBaseInterface>> getImpliedFeatures() {
      return Collections.emptySet();
    }

    @Retention(RetentionPolicy.RUNTIME)
    @Inherited
    @TesterAnnotation
    @interface Require {
      ExampleBaseFeature[] value() default {};

  private final Set<Feature<? super List>> implied;

  ListFeature(Feature<? super List>... implied) {
    this.implied = Helpers.copyToSet(implied);
  }

  @Override
  public Set<Feature<? super List>> getImpliedFeatures() {
    return implied;
  }

  @Retention(RetentionPolicy.RUNTIME)
  @Inherited
  @TesterAnnotation
  public @interface Require {
    ListFeature[] value() default {};