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