return () -> value;
  }

  /**
   * Creates an {@link AsyncCallable} from a {@link Callable}.
   *
   * <p>The {@link AsyncCallable} returns the {@link ListenableFuture} resulting from {@link
   * ListeningExecutorService#submit(Callable)}.
   *
   * @since 20.0
   */
  @J2ktIncompatible
  @GwtIncompatible
  public static <T extends @Nullable Object> AsyncCallable<T> asAsyncCallable(

    String expected = "MyCallableString";
    Callable<String> callable = () -> expected;

    AsyncCallable<String> asyncCallable =
        Callables.asAsyncCallable(callable, newDirectExecutorService());

    ListenableFuture<String> future = asyncCallable.call();
    assertThat(future.get()).isEqualTo(expected);
  }

  @J2ktIncompatible
  @GwtIncompatible

    String expected = "MyCallableString";
    Callable<String> callable = () -> expected;

    AsyncCallable<String> asyncCallable =
        Callables.asAsyncCallable(callable, newDirectExecutorService());

    ListenableFuture<String> future = asyncCallable.call();
    assertThat(future.get()).isEqualTo(expected);
  }

  @J2ktIncompatible
  @GwtIncompatible

   * callable} or a callable that has begun to execute, but if the output future is cancelled before
   * {@link AsyncCallable#call()} is invoked, {@link AsyncCallable#call()} will not be invoked.
   */
  public <T extends @Nullable Object> ListenableFuture<T> submitAsync(
      AsyncCallable<T> callable, Executor executor) {
    checkNotNull(callable);
    checkNotNull(executor);

    @SuppressWarnings("AnonymousToLambda") // We want an instance that can be GCed.
    AsyncCallable<Long> combiner =
        new AsyncCallable<Long>() {
          @Override
          public ListenableFuture<Long> call() {
            return SettableFuture.create();
          }
        };
    WeakReference<AsyncCallable<Long>> combinerRef = new WeakReference<>(combiner);

    ListenableFuture<Long> unused =

    @SuppressWarnings("AnonymousToLambda") // We want an instance that can be GCed.
    AsyncCallable<Long> combiner =
        new AsyncCallable<Long>() {
          @Override
          public ListenableFuture<Long> call() {
            return SettableFuture.create();
          }
        };
    WeakReference<AsyncCallable<Long>> combinerRef = new WeakReference<>(combiner);

    ListenableFuture<Long> unused =

    }

    void stop() {
      stopLatch.countDown();
    }

    boolean isRunning() {
      return running;
    }
  }

  private static final class TestCallable implements AsyncCallable<@Nullable Void> {

    private final ListenableFuture<@Nullable Void> future;
    private boolean called = false;

    private TestCallable(ListenableFuture<@Nullable Void> future) {
      this.future = future;