@Override
        @Nullable N visitNext(Deque<Iterator<? extends N>> horizon) {
          Iterator<? extends N> top = horizon.getFirst();
          if (top.hasNext()) {
            return checkNotNull(top.next());
          }
          horizon.removeFirst();
          return null;
        }
      };
    }

    final Iterator<N> breadthFirst(Iterator<? extends N> startNodes) {

        : newArrayList(elements.iterator());
  }

  /**
   * Creates a <i>mutable</i> {@code ArrayList} instance containing the given elements; a very thin
   * shortcut for creating an empty list and then calling {@link Iterators#addAll}.
   *
   * <p><b>Note:</b> if mutability is not required and the elements are non-null, use {@link
   * ImmutableList#copyOf(Iterator)} instead.
   */

        : newArrayList(elements.iterator());
  }

  /**
   * Creates a <i>mutable</i> {@code ArrayList} instance containing the given elements; a very thin
   * shortcut for creating an empty list and then calling {@link Iterators#addAll}.
   *
   * <p><b>Note:</b> if mutability is not required and the elements are non-null, use {@link
   * ImmutableList#copyOf(Iterator)} instead.
   */

   * For example, two graphs may be considered equal even if one allows self-loops and the other
   * doesn't. Additionally, the order in which nodes or edges are added to the graph, and the order
   * in which they are iterated over, are irrelevant.
   *
   * <p>A reference implementation of this is provided by {@link AbstractValueGraph#equals(Object)}.
   */
  @Override
  boolean equals(@Nullable Object object);

  /**

   * For example, two graphs may be considered equal even if one allows self-loops and the other
   * doesn't. Additionally, the order in which nodes or edges are added to the graph, and the order
   * in which they are iterated over, are irrelevant.
   *
   * <p>A reference implementation of this is provided by {@link AbstractGraph#equals(Object)}.
   */
  @Override
  boolean equals(@Nullable Object object);

  /**

  }

  /**
   * Returns an immutable multimap containing the specified entries. The returned multimap iterates
   * over keys in the order they were first encountered in the input, and the values for each key
   * are iterated in the order they were encountered.
   *
   * @throws NullPointerException if any key, value, or entry is null
   * @since 19.0
   */

   * close the open underlying stream.
   *
   * <p>Note: The input {@code Iterator} will be copied to an {@code ImmutableList} when this method
   * is called. This will fail if the iterator is infinite and may cause problems if the iterator
   * eagerly fetches data for each source when iterated (rather than producing sources that only

  }

  /**
   * Returns an immutable multimap containing the specified entries. The returned multimap iterates
   * over keys in the order they were first encountered in the input, and the values for each key
   * are iterated in the order they were encountered.
   *
   * @throws NullPointerException if any key, value, or entry is null
   * @since 19.0
   */

  }

  @Override
  UnmodifiableIterator<Entry<K, V>> entryIterator() {
    return new UnmodifiableIterator<Entry<K, V>>() {
      final Iterator<? extends Entry<K, ? extends ImmutableCollection<V>>> asMapItr =
          map.entrySet().iterator();
      @Nullable K currentKey = null;
      Iterator<V> valueItr = emptyIterator();

      @Override
      public boolean hasNext() {
        return valueItr.hasNext() || asMapItr.hasNext();

   * For example, two networks may be considered equal even if one allows parallel edges and the
   * other doesn't. Additionally, the order in which nodes or edges are added to the network, and
   * the order in which they are iterated over, are irrelevant.
   *
   * <p>A reference implementation of this is provided by {@link AbstractNetwork#equals(Object)}.
   */
  @Override
  boolean equals(@Nullable Object object);

  /**