*
     * @since 20.0 (present with return type {@link Iterator} since 2.0)
     */
    @Override
    public abstract UnmodifiableIterator<E> iterator();

    @Override
    @SuppressWarnings("EqualsHashCode") // same semantics
    public boolean equals(@Nullable Object object) {
      if (object == this) {
        return true;
      }
      if (!(object instanceof Set)) {
        return false;
      }