this.header = new AvlNode<>();
    successor(header, header);
    this.rootReference = new Reference<>();
  }

  /** A function which can be summed across a subtree. */
  private enum Aggregate {
    SIZE {
      @Override
      int nodeAggregate(AvlNode<?> node) {
        return node.elemCount;
      }

      @Override
      long treeAggregate(@CheckForNull AvlNode<?> root) {

   *
   * @since 14.0
   */
  public static HashFunction adler32() {
    return ChecksumType.ADLER_32.hashFunction;
  }

  @Immutable
  enum ChecksumType implements ImmutableSupplier<Checksum> {
    CRC_32("Hashing.crc32()") {
      @Override
      public Checksum get() {
        return new CRC32();
      }
    },
    ADLER_32("Hashing.adler32()") {

 */
@GwtCompatible(serializable = true, emulated = true)
@SuppressWarnings("serial") // we're overriding default serialization
@ElementTypesAreNonnullByDefault
final class ImmutableEnumSet<E extends Enum<E>> extends ImmutableSet<E> {
  static <E extends Enum<E>> ImmutableSet<E> asImmutable(EnumSet<E> set) {
    switch (set.size()) {
      case 0:
        return ImmutableSet.of();
      case 1:

        assertThat(pd.getElementClassOfCollection(), is(sameClass(Enum.class)));

        pcd = pd.getParameterizedClassDesc();
        assertThat(pcd.getRawClass(), is(sameClass(Set.class)));
        assertThat(pcd.getArguments().length, is(1));
        assertThat(pcd.getArguments()[0].getRawClass(), is(sameClass(Enum.class)));
    }

    /**
     * @throws Exception
     */

/**
 * Request used to build a {@link org.apache.maven.api.Project} using
 * the {@link ProjectBuilder} service.
 *
 * TODO: replace ModelRepositoryHolder with just the enum for the strategy
 * TODO: replace validation level with an enum (though, we usually need just a boolean)
 *
 * @since 4.0.0
 */
@Experimental
@Immutable
public interface ModelBuilderRequest {

    /**

      List<E> elements = new ArrayList<>();
      Helpers.addAll(elements, previousElements);
      Helpers.addAll(elements, Helpers.reverse(nextElements));
      return elements;
    }
  }

  public enum KnownOrder {
    KNOWN_ORDER,
    UNKNOWN_ORDER
  }

  @SuppressWarnings("unchecked") // TODO(cpovirk): Stop using arrays.
  AbstractIteratorTester(
      int steps,

          queueIterator.remove();
        }
      }
      assertThat(remaining).isEqualTo(0);
      assertIntact(queue);
      assertThat(queue).containsExactlyElementsIn(elements);
    }
  }

  private enum Element {
    ONE,
    TWO,
    THREE,
    FOUR,
    FIVE;
  }

  public void testRandomAddsAndRemoves_duplicateElements() {
    Random random = new Random(0);

    map.put(3, 20);
    map.put(1, 30);
    TreeMap<Integer, Integer> copy = Maps.newTreeMap(map);
    assertEquals(copy, map);
    assertSame(copy.comparator(), map.comparator());
  }

  public enum SomeEnum {
    SOME_INSTANCE
  }

  public void testEnumMap() {
    EnumMap<SomeEnum, Integer> map = Maps.newEnumMap(SomeEnum.class);
    assertEquals(Collections.emptyMap(), map);

    map.put(3, 20);
    map.put(1, 30);
    TreeMap<Integer, Integer> copy = Maps.newTreeMap(map);
    assertEquals(copy, map);
    assertSame(copy.comparator(), map.comparator());
  }

  public enum SomeEnum {
    SOME_INSTANCE
  }

  public void testEnumMap() {
    EnumMap<SomeEnum, Integer> map = Maps.newEnumMap(SomeEnum.class);
    assertEquals(Collections.emptyMap(), map);

 * can be used as keys.
 *
 * @since 4.0.0
 */
@Experimental
public interface ExtensibleEnum {

    /**
     * The {@code id} uniquely represents a value for this extensible enum.
     * This id should be used to compute the equality and hash code for the instance.
     *
     * @return the id
     */
    @Nonnull
    String id();