* supports it.
   */
  public static <T extends @Nullable Object> Iterator<T> concat(
      Iterator<? extends T> a, Iterator<? extends T> b) {
    checkNotNull(a);
    checkNotNull(b);
    return concat(consumingForArray(a, b));
  }

  /**
   * Combines three iterators into a single iterator. The returned iterator iterates across the

   * supports it.
   */
  public static <T extends @Nullable Object> Iterator<T> concat(
      Iterator<? extends T> a, Iterator<? extends T> b) {
    checkNotNull(a);
    checkNotNull(b);
    return concat(consumingForArray(a, b));
  }

  /**
   * Combines three iterators into a single iterator. The returned iterator iterates across the

    assertEquals("d", itr2.next());
    assertEquals("c", itr2.next());
  }

  public void testConcatPartiallyAdvancedFirst() {
    Iterator<String> itr1 = Iterators.concat(singletonIterator("a"), Iterators.forArray("b", "c"));
    assertEquals("a", itr1.next());
    assertEquals("b", itr1.next());
    Iterator<String> itr2 = Iterators.concat(itr1, singletonIterator("d"));

      protected Iterator<Integer> newTargetIterator() {
        return Iterators.concat();
      }
    }.test();
  }

  @GwtIncompatible // slow (~5s)
  public void testConcatOneEmptyIteratorYieldsEmpty() {
    new EmptyIteratorTester() {
      @Override
      protected Iterator<Integer> newTargetIterator() {
        return Iterators.concat(iterateOver());
      }
    }.test();
  }

    EntryTransformer<String, String, String> concat =
        new EntryTransformer<String, String, String>() {
          @Override
          public String transformEntry(String key, String value) {
            return key + value;
          }
        };
    Map<String, String> transformed = transformEntries(map, concat);

    assertEquals(ImmutableMap.of("a", "a4", "b", "b9"), transformed);

    EntryTransformer<String, String, String> concat =
        new EntryTransformer<String, String, String>() {
          @Override
          public String transformEntry(String key, String value) {
            return key + value;
          }
        };
    Map<String, String> transformed = transformEntries(map, concat);

    assertEquals(ImmutableMap.of("a", "a4", "b", "b9"), transformed);

                boolean recursive) {
            List<Callable<InterimResult>> tasks = pomFiles.stream()
                    .map(pomFile -> ((Callable<InterimResult>)
                            () -> build(projectIndex, pomFile, concat(aggregatorFiles, pomFile), root, recursive)))
                    .collect(Collectors.toList());
            try {
                List<Future<InterimResult>> futures = executor.invokeAll(tasks);

// order, and type) the op declaration.  For example, the "Concat" op
// has registration:
//   REGISTER_OP("Concat")
//       .Input("concat_dim: int32")
//       .Input("values: N * T")
//       .Output("output: T")
//       .Attr("N: int >= 2")
//       .Attr("T: type");
// that defines two inputs, "concat_dim" and "values" (in that order).

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