}
      };

  private static <V> AsyncFunction<V, V> asyncIdentity() {
    return new AsyncFunction<V, V>() {
      @Override
      public ListenableFuture<V> apply(V input) {
        return immediateFuture(input);
      }
    };
  }

  private static <I, O> AsyncFunction<I, O> tagged(
      final String toString, final AsyncFunction<I, O> function) {
    return new AsyncFunction<I, O>() {

        }
      };

  private static <V> AsyncFunction<V, V> asyncIdentity() {
    return new AsyncFunction<V, V>() {
      @Override
      public ListenableFuture<V> apply(V input) {
        return immediateFuture(input);
      }
    };
  }

  private static <I, O> AsyncFunction<I, O> tagged(
      final String toString, final AsyncFunction<I, O> function) {
    return new AsyncFunction<I, O>() {

      Class<X> exceptionType,
      final AsyncClosingFunction<? super X, W> fallback,
      Executor executor) {
    checkNotNull(fallback);
    AsyncFunction<X, W> asyncFunction =
        new AsyncFunction<X, W>() {
          @Override
          public ListenableFuture<W> apply(X exception) throws Exception {
            return closeables.applyAsyncClosingFunction(fallback, exception);
          }