public static IObservable <TOutput> ExecuteSequentally <TInput, TOutput>(this RxAction <TInput, TOutput> action,
                                                                          params TInput[] inputs)
 => action.executeSequentally(inputs);
 public static IObservable <Unit> ExecuteSequentally(this RxAction <Unit, Unit> action, int times)
 => action.executeSequentally(Enumerable.Range(0, times).Select(_ => default(Unit)));
 public static IObservable <TOutput> ExecuteSequentally <TInput, TOutput>(this RxAction <TInput, TOutput> action,
                                                                          IEnumerable <TInput> inputs)
 => action.executeSequentally(inputs);