@Override
        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) {

        @Override
        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) {

      }
    }
    size--;
  }

  /** Removes all nodes for the specified key. */
  private void removeAllNodes(@ParametricNullness K key) {
    Iterators.clear(new ValueForKeyIterator(key));
  }

  /** An {@code Iterator} over all nodes. */
  private class NodeIterator implements ListIterator<Entry<K, V>> {
    int nextIndex;
    @CheckForNull Node<K, V> next;
    @CheckForNull Node<K, V> current;

      return original.size();
    }

    @Override
    public Collection<V> values() {
      return original.values();
    }

    @Override
    Iterator<Cell<C, R, V>> cellIterator() {
      return Iterators.transform(original.cellSet().iterator(), Tables::transposeCell);
    }

    @Override
    Spliterator<Cell<C, R, V>> cellSpliterator() {

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

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

      iterator.remove();
    }
  }

  /** ValueIterator build on top of EntryIterator. */
  final class ValueIterator implements Iterator<V> {
    private EntryIterator iterator;

    ValueIterator() {
      iterator = new EntryIterator();
    }

    @Override
    public boolean hasNext() {
      return iterator.hasNext();
    }

    @Override
    public V next() {

      return original.size();
    }

    @Override
    public Collection<V> values() {
      return original.values();
    }

    @Override
    Iterator<Cell<C, R, V>> cellIterator() {
      return Iterators.transform(original.cellSet().iterator(), Tables::transposeCell);
    }
  }

  private static <
          R extends @Nullable Object, C extends @Nullable Object, V extends @Nullable Object>

    return new Iterable<T>() {
      @Override
      public Iterator<T> iterator() {
        return new AbstractIterator<T>() {
          private final Iterator<? extends Optional<? extends T>> iterator =
              checkNotNull(optionals.iterator());

          @Override
          @CheckForNull
          protected T computeNext() {
            while (iterator.hasNext()) {

      }
    }
    size--;
  }

  /** Removes all nodes for the specified key. */
  private void removeAllNodes(@ParametricNullness K key) {
    Iterators.clear(new ValueForKeyIterator(key));
  }

  /** An {@code Iterator} over all nodes. */
  private class NodeIterator implements ListIterator<Entry<K, V>> {
    int nextIndex;
    @CheckForNull Node<K, V> next;
    @CheckForNull Node<K, V> current;

   * exception for its correctness: <i>the fail-fast behavior of iterators should be used only to
   * detect bugs.</i>
   *
   * @return an iterator over the elements contained in this collection
   */
  @Override
  public Iterator<E> iterator() {
    return new QueueIterator();
  }

  @Override
  public void clear() {
    for (int i = 0; i < size; i++) {