}

    @Override
    public boolean removeAll(final Collection<?> collection) {
      return removeIf(collection::contains);
    }

    @Override
    public boolean retainAll(final Collection<?> collection) {
      return removeIf(element -> !collection.contains(element));
    }

    @Override

   */
  public static boolean elementsEqual(Iterable<?> iterable1, Iterable<?> iterable2) {
    if (iterable1 instanceof Collection && iterable2 instanceof Collection) {
      Collection<?> collection1 = (Collection<?>) iterable1;
      Collection<?> collection2 = (Collection<?>) iterable2;
      if (collection1.size() != collection2.size()) {
        return false;
      }
    }

   * </ol>
   *
   * @param collection the presumed-immutable collection
   * @param sampleElement an element of the same type as that contained by {@code collection}.
   *     {@code collection} may or may not have {@code sampleElement} as a member.
   */
  public static <E extends @Nullable Object> void assertCollectionIsUnmodifiable(
      Collection<E> collection, E sampleElement) {
    Collection<E> siblingCollection = new ArrayList<>();

   */
  Collection<V> values();

  /**
   * Returns a view collection of all key-value pairs contained in this multimap, as {@link Entry}
   * instances.
   *
   * <p>Changes to the returned collection or the entries it contains will update the underlying
   * multimap, and vice versa. However, <i>adding</i> to the returned collection is not possible.
   */

  private static final Feature<?>[] COLLECTION_FEATURES = {
    CollectionSize.ANY, CollectionFeature.ALLOWS_NULL_QUERIES
  };

  private static final Feature<?>[] COLLECTION_FEATURES_ORDER = {
    CollectionSize.ANY, CollectionFeature.KNOWN_ORDER, CollectionFeature.ALLOWS_NULL_QUERIES
  };

  private static final Feature<?>[] COLLECTION_FEATURES_REMOVE = {

    assertEquals(Collections.singleton("a"), set);
  }

  public void testCopyOf_collection_oneElementRepeated() {
    Collection<String> c = MinimalCollection.of("a", "a", "a");
    Set<String> set = copyOf(c);
    assertEquals(Collections.singleton("a"), set);
  }

  public void testCopyOf_collection_general() {
    Collection<String> c = MinimalCollection.of("a", "b", "a");
    Set<String> set = copyOf(c);

    assertEquals(Collections.singleton("a"), set);
  }

  public void testCopyOf_collection_oneElementRepeated() {
    Collection<String> c = MinimalCollection.of("a", "a", "a");
    Set<String> set = copyOf(c);
    assertEquals(Collections.singleton("a"), set);
  }

  public void testCopyOf_collection_general() {
    Collection<String> c = MinimalCollection.of("a", "b", "a");
    Set<String> set = copyOf(c);

public class CollectionRetainAllTester<E> extends AbstractCollectionTester<E> {

  /** A collection of elements to retain, along with a description for use in failure messages. */
  private class Target {
    private final Collection<E> toRetain;
    private final String description;

    private Target(Collection<E> toRetain, String description) {
      this.toRetain = toRetain;
      this.description = description;
    }

    StringBuilder sb = newStringBuilderForCollection(collection.size()).append('[');
    boolean first = true;
    for (Object o : collection) {
      if (!first) {
        sb.append(", ");
      }
      first = false;
      if (o == collection) {
        sb.append("(this Collection)");
      } else {
        sb.append(o);
      }
    }
    return sb.append(']').toString();

  }

  public void testCopyOf_collection_empty() {
    // "<String>" is required to work around a javac 1.5 bug.
    Collection<String> c = MinimalCollection.<String>of();
    Multiset<String> multiset = ImmutableSortedMultiset.copyOf(c);
    assertTrue(multiset.isEmpty());
  }

  public void testCopyOf_collection_oneElement() {
    Collection<String> c = MinimalCollection.of("a");