return Absent.withType();
  }

  /**
   * Returns an {@code Optional} instance containing the given non-null reference. To have {@code
   * null} treated as {@link #absent}, use {@link #fromNullable} instead.
   *
   * <p><b>Comparison to {@code java.util.Optional}:</b> no differences.
   *
   * @throws NullPointerException if {@code reference} is null
   */
  public static <T> Optional<T> of(T reference) {

    return Absent.withType();
  }

  /**
   * Returns an {@code Optional} instance containing the given non-null reference. To have {@code
   * null} treated as {@link #absent}, use {@link #fromNullable} instead.
   *
   * <p><b>Comparison to {@code java.util.Optional}:</b> no differences.
   *
   * @throws NullPointerException if {@code reference} is null
   */
  public static <T> Optional<T> of(T reference) {

    List<Set<Optional<?>>> optionalSets = Lists.newArrayListWithExpectedSize(sets.length);
    for (Set<?> set : sets) {
      Set<Optional<?>> optionalSet =
          Sets.newLinkedHashSet(Iterables.transform(set, Optional::fromNullable));
      optionalSets.add(optionalSet);
    }
    Set<List<Optional<?>>> cartesianProduct = Sets.cartesianProduct(optionalSets);

  }

  public void testFromNullable() {
    Optional<String> optionalName = Optional.fromNullable("bob");
    assertThat(optionalName.get()).isEqualTo("bob");
  }

  public void testFromNullable_null() {
    // not promised by spec, but easier to test
    assertThat(Optional.fromNullable(null)).isSameInstanceAs(Optional.absent());
  }

  public void testIsPresent_no() {