private static <X extends Throwable, V> AsyncFunctionSpy<X, V> spy(AsyncFunction<X, V> delegate) {
    return new AsyncFunctionSpy<>(delegate);
  }

  private static <X extends Throwable, V> AsyncFunction<X, V> unexpectedAsyncFunction() {
    return new AsyncFunction<X, V>() {
      @Override
      public ListenableFuture<V> apply(X t) {
        throw new AssertionError("Unexpected fallback", t);
      }
    };
  }

  private static <X extends Throwable, V> AsyncFunctionSpy<X, V> spy(AsyncFunction<X, V> delegate) {
    return new AsyncFunctionSpy<>(delegate);
  }

  private static <X extends Throwable, V> AsyncFunction<X, V> unexpectedAsyncFunction() {
    return new AsyncFunction<X, V>() {
      @Override
      public ListenableFuture<V> apply(X t) {
        throw new AssertionError("Unexpected fallback", t);
      }
    };
  }