return t -> {
      throw new AssertionError("Unexpected fallback", t);
    };
  }

  private static class AsyncFunctionSpy<X extends Throwable, V> implements AsyncFunction<X, V> {
    private int count;
    private final AsyncFunction<X, V> delegate;

    AsyncFunctionSpy(AsyncFunction<X, V> delegate) {
      this.delegate = delegate;
    }

    @Override

    return t -> {
      throw new AssertionError("Unexpected fallback", t);
    };
  }

  private static class AsyncFunctionSpy<X extends Throwable, V> implements AsyncFunction<X, V> {
    private int count;
    private final AsyncFunction<X, V> delegate;

    AsyncFunctionSpy(AsyncFunction<X, V> delegate) {
      this.delegate = delegate;
    }

    @Override