@Beta // TODO: b/288085449 - Remove.
  public static <E> Collector<E, ?, ImmutableSet<E>> toImmutableSet() {
    throw new UnsupportedOperationException();
  }

  /**
   * Not supported. Use {@link #naturalOrder}, which offers better type-safety, instead. This method
   * exists only to hide {@link ImmutableSet#builder} from consumers of {@code ImmutableSortedSet}.
   *

@SuppressWarnings("serial") // we're overriding default serialization
@ElementTypesAreNonnullByDefault
public abstract class ImmutableSortedSet<E> extends ImmutableSet.CachingAsList<E>
    implements NavigableSet<E>, SortedIterable<E> {
  static final int SPLITERATOR_CHARACTERISTICS =
      ImmutableSet.SPLITERATOR_CHARACTERISTICS | Spliterator.SORTED;

  /**
   * Returns a {@code Collector} that accumulates the input elements into a new {@code

  // In the non-emulated source, this is in ImmutableSortedSetFauxverideShim,
  // which overrides ImmutableSet & which ImmutableSortedSet extends.
  // It is necessary here because otherwise the builder() method
  // would be inherited from the emulated ImmutableSet.
  // TODO(cpovirk): should we be including other methods from the shim here and
  // in ImmutableSortedMap?
  @Deprecated