Stream<String> streamB = Stream.of("b").onClose(closeCountB::incrementAndGet);
    Stream<String> concatenated =
        Stream.<Stream<String>>of(Stream.of("a"), streamB, Stream.empty(), Stream.of("c", "d"))
            .reduce(Stream.empty(), Stream::concat);
    assertThat(concatenated).containsExactly("a", "b", "c", "d").inOrder();
    concatenated.close();
    assertThat(closeCountB.get()).isEqualTo(1);
  }

  }

  /**
   * Concatenates multiple {@link ByteSource} instances into a single source. Streams returned from
   * the source will contain the concatenated data from the streams of the underlying sources.
   *
   * <p>Only one underlying stream will be open at a time. Closing the concatenated stream will
   * close the open underlying stream.
   *
   * @param sources the sources to concatenate

  }

  /**
   * Concatenates multiple {@link CharSource} instances into a single source. Streams returned from
   * the source will contain the concatenated data from the streams of the underlying sources.
   *
   * <p>Only one underlying stream will be open at a time. Closing the concatenated stream will
   * close the open underlying stream.
   *
   * @param sources the sources to concatenate

   * RandomAccess} support, then this operation is guaranteed to be {@code O(1)}.
   *
   * <p><b>{@code Stream} equivalent:</b> {@link Streams#findLast Streams.findLast(stream).get()}
   *
   * @return the last element of {@code iterable}
   * @throws NoSuchElementException if the iterable is empty
   */
  @ParametricNullness

   * input element is mapped to a key and a stream of values, each of which are put into the
   * resulting {@code Multimap}, in the encounter order of the stream and the encounter order of the
   * streams of values.
   *
   * <p>Example:
   *
   * <pre>{@code
   * static final ImmutableSetMultimap<Character, Character> FIRST_LETTER_MULTIMAP =
   *     Stream.of("banana", "apple", "carrot", "asparagus", "cherry")

   *
   * <p>For example:
   *
   * <pre>{@code
   * Stream.of("foo", "quux", "banana", "elephant")
   *     .collect(least(2, comparingInt(String::length)))
   * // returns {"foo", "quux"}
   * }</pre>
   *
   * <p>This {@code Collector} uses O(k) memory and takes expected time O(n) (worst-case O(n log
   * k)), as opposed to e.g. {@code Stream.sorted(comparator).limit(k)}, which currently takes O(n

   * iterator supports it.
   *
   * <p><b>{@code Stream} equivalent:</b> to concatenate an arbitrary number of streams, use {@code
   * Stream.of(stream1, stream2, ...).flatMap(s -> s)}. If the sources are iterables, use {@code
   * Stream.of(iter1, iter2, ...).flatMap(Streams::stream)}.
   *
   * @throws NullPointerException if any of the provided iterables is {@code null}
   * @since 20.0

 * well.
 *
 * <h3>Java 8+ users</h3>
 *
 * <p>If you are using Java 8+, this class is now obsolete. Most of its functionality is now
 * provided by {@link java.util.stream.Stream Stream} and by {@link Comparator} itself, and the rest
 * can now be found as static methods in our new {@link Comparators} class. See each method below

   * RandomAccess} support, then this operation is guaranteed to be {@code O(1)}.
   *
   * <p><b>{@code Stream} equivalent:</b> {@link Streams#findLast Streams.findLast(stream).get()}
   *
   * @return the last element of {@code iterable}
   * @throws NoSuchElementException if the iterable is empty
   */
  @ParametricNullness

 *   <li>{@link Maps}
 *   <li>{@link MoreCollectors}
 *   <li>{@link Multimaps}
 *   <li>{@link Multisets}
 *   <li>{@link ObjectArrays}
 *   <li>{@link Queues}
 *   <li>{@link Sets}
 *   <li>{@link Streams}
 *   <li>{@link Tables}
 * </ul>
 *
 * <h2>Abstract implementations</h2>
 *
 * <ul>
 *   <li>{@link AbstractIterator}
 *   <li>{@link AbstractSequentialIterator}
 *   <li>{@link UnmodifiableIterator}