return !queue.isEmpty();
    }

    @Override
    @ParametricNullness
    public T next() {
      PeekingIterator<T> nextIter = queue.remove();
      T next = nextIter.next();
      if (nextIter.hasNext()) {
        queue.add(nextIter);
      }
      return next;
    }
  }

  private static class ConcatenatedIterator<T extends @Nullable Object> implements Iterator<T> {

      return !queue.isEmpty();
    }

    @Override
    @ParametricNullness
    public T next() {
      PeekingIterator<T> nextIter = queue.remove();
      T next = nextIter.next();
      if (nextIter.hasNext()) {
        queue.add(nextIter);
      }
      return next;
    }
  }

  private static class ConcatenatedIterator<T extends @Nullable Object> implements Iterator<T> {

  auto b = ops::Placeholder(root.WithOpName("B"), DT_FLOAT);
  auto c = ops::Add(root.WithOpName("C"), a, b);
  auto enter = ops::internal::Enter(root, c, "aframe");
  auto next_iter = ops::NextIteration(root, enter);
  auto exit = ops::internal::Exit(root, next_iter);
  auto d = ops::Add(root.WithOpName("D"), c, exit);

  std::unique_ptr<Graph> graph(new Graph(OpRegistry::Global()));
  TF_EXPECT_OK(root.ToGraph(graph.get()));