* Collections#singleton(Object)} and {@link java.util.LinkedHashSet} for empty, singleton and
 * regular sets respectively. For the sorted sets, it's a thin wrapper around {@link
 * java.util.TreeSet}.
 *
 * @see ImmutableSortedSet
 * @author Hayward Chan
 */
@ElementTypesAreNonnullByDefault
@SuppressWarnings("serial") // Serialization only done in GWT.
public abstract class ImmutableSet<E> extends ImmutableCollection<E> implements Set<E> {