/**
   * 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 emptySet();
  }

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

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

  MapFeature(Feature<? super Map>... implied) {
    this.implied = copyToSet(implied);
  }

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

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

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

  SetFeature(Feature<? super Set>... implied) {
    this.implied = 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 Multimap>> implied;

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

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

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

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

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

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

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

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

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

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

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

    Queue<Feature<?>> queue = new ArrayDeque<>(features);
    while (!queue.isEmpty()) {
      Feature<?> feature = queue.remove();
      for (Feature<?> implied : feature.getImpliedFeatures()) {
        if (features.add(implied)) {
          queue.add(implied);
        }
      }
    }
    return features;
  }

  /**

    Queue<Feature<?>> queue = new ArrayDeque<>(features);
    while (!queue.isEmpty()) {
      Feature<?> feature = queue.remove();
      for (Feature<?> implied : feature.getImpliedFeatures()) {
        if (features.add(implied)) {
          queue.add(implied);
        }
      }
    }
    return features;
  }

  /**

    // gives a predictable order of test suites.
    this.implied = copyToSet(implied);
    this.numElements = null;
  }

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

  public int getNumElements() {
    if (numElements == null) {
      throw new IllegalStateException(
          "A compound CollectionSize doesn't specify a number of elements.");