return ImmutableSet.of();
      }
    }
  }

  /**
   * Returns a {@code Collector} that accumulates the input elements into a new {@code ImmutableSet}
   * with an implementation specialized for enums. Unlike {@link ImmutableSet#toImmutableSet}, the
   * resulting set will iterate over elements in their enum definition order, not encounter order.
   *
   * @since 33.2.0 (available since 21.0 in guava-jre)
   */