assertThat(Optional.presentInstances(optionals)).containsExactly("a", "b", "c").inOrder();
  }

  public void testPresentInstances_allAbsent() {
    List<Optional<Object>> optionals = ImmutableList.of(Optional.absent(), Optional.absent());
    assertThat(Optional.presentInstances(optionals)).isEmpty();
  }

  public void testPresentInstances_somePresent() {

  }

  private static final TreeTraverser<BinaryNode> VIEWER =
      new TreeTraverser<BinaryNode>() {
        @Override
        public Iterable<BinaryNode> children(BinaryNode root) {
          return Optional.presentInstances(ImmutableList.of(root.left, root.right));
        }
      };

  enum Traversal {
    PRE_ORDER {
      @Override
      <T> Iterable<T> view(T root, TreeTraverser<T> viewer) {

  }

  private static final TreeTraverser<BinaryNode> VIEWER =
      new TreeTraverser<BinaryNode>() {
        @Override
        public Iterable<BinaryNode> children(BinaryNode root) {
          return Optional.presentInstances(ImmutableList.of(root.left, root.right));
        }
      };

  enum Traversal {
    PRE_ORDER {
      @Override
      <T> Iterable<T> view(T root, TreeTraverser<T> viewer) {

   *
   * <p><b>Java 9 users:</b> use {@code optionals.stream().flatMap(Optional::stream)} instead.
   *
   * @since 11.0 (generics widened in 13.0)
   */
  public static <T> Iterable<T> presentInstances(
      final Iterable<? extends Optional<? extends T>> optionals) {
    checkNotNull(optionals);
    return new Iterable<T>() {
      @Override
      public Iterator<T> iterator() {