docs: public Observable<Mail> addMailWithDelay(final Mail mail) {
      return Observable.defer(new Func0<Observable<Mail>>() {
        @Override public Observable<Mail> call() {
  
          delay();
          Observable o = checkExceptions();
          if (o != null) {
            return o;
          }
          return Observable.just(mail);
        }
      }).flatMap(new Func1<Mail, Observable<Mail>>() {
        @Override public Observable<Mail> call(Mail mail) {
          return addMail(mail);
        }
      });
    }