code: @SuppressWarnings("unchecked")
    public static <R> Fallback<R> ofAsync(CheckedFunction<ExecutionAttemptedEvent<? extends R>, ? extends R> fallback) {
      return new Fallback<>(Assert.notNull((CheckedFunction) fallback, "fallback"), true);
    }
docs: Configures the {@code fallback} to be executed asynchronously if execution fails. The {@code fallback} applies an
  {@link ExecutionAttemptedEvent}.
  
  @throws NullPointerException if {@code fallback} is null