public abstract class ImmutableSet<E> extends ImmutableCollection<E> implements Set<E> {
  ImmutableSet() {}

  public static <E> Collector<E, ?, ImmutableSet<E>> toImmutableSet() {
    return CollectCollectors.toImmutableSet();
  }

  // Casting to any type is safe because the set will never hold any elements.
  @SuppressWarnings({"unchecked"})
  public static <E> ImmutableSet<E> of() {