CollectionSize.SEVERAL,
                CollectionFeature.SERIALIZABLE,
                CollectionFeature.ALLOWS_NULL_VALUES)
            .createTestSuite());

    final Function<String, String> removeFirst = new RemoveFirstFunction();

    suite.addTest(
        ListTestSuiteBuilder.using(
                new TestStringListGenerator() {
                  @Override

             */
            requireNonNull(element);
            if (visited.add(element)) {
              return element;
            }
          }
          horizon.removeFirst();
          return null;
        }
      };
    }

    static <N> Traversal<N> inTree(SuccessorsFunction<N> tree) {
      return new Traversal<N>(tree) {
        @CheckForNull
        @Override

                CollectionSize.SEVERAL,
                CollectionFeature.SERIALIZABLE,
                CollectionFeature.ALLOWS_NULL_VALUES)
            .createTestSuite());

    final Function<String, String> removeFirst = new RemoveFirstFunction();

    suite.addTest(
        ListTestSuiteBuilder.using(
                new TestStringListGenerator() {
                  @Override

   */
  @Override
  public void close() throws IOException {
    Throwable throwable = thrown;

    // close closeables in LIFO order
    while (!stack.isEmpty()) {
      Closeable closeable = stack.removeFirst();
      try {
        closeable.close();
      } catch (Throwable e) {
        if (throwable == null) {
          throwable = e;
        } else {
          suppressor.suppress(closeable, throwable, e);

    return poll();
  }

  /**
   * Removes and returns the least element of this queue.
   *
   * @throws NoSuchElementException if the queue is empty
   */
  @CanIgnoreReturnValue
  public E removeFirst() {
    return remove();
  }

  /**
   * Retrieves, but does not remove, the least element of this queue, or returns {@code null} if the
   * queue is empty.
   */
  @CheckForNull