}
    return null;
  }

  /**
   * An {@link AbstractTransformFuture} that delegates to an {@link AsyncFunction} and {@link
   * #setFuture(ListenableFuture)}.
   */
  private static final class AsyncTransformFuture<
          I extends @Nullable Object, O extends @Nullable Object>
      extends AbstractTransformFuture<
          I, O, AsyncFunction<? super I, ? extends O>, ListenableFuture<? extends O>> {