code: protected OptionalThing<Locale> prepareReceiverLocale(Postcard postcard) {
          final OptionalThing<Locale> receiverLocale = postcard.getReceiverLocale();
          if (receiverLocale.isPresent()) {
              return receiverLocale;
          } else {
              if (receiverLocaleAssist != null) {
                  final OptionalThing<Locale> assistedLocale = receiverLocaleAssist.assist(postcard);
                  if (assistedLocale == null) {
                      String msg = "Cannot return null as optional type: receiverLocaleAssist=" + receiverLocaleAssist + ", " + postcard;
                      throw new SMailIllegalStateException(msg);
                  }
                  assistedLocale.ifPresent(locale -> {
                      postcard.asReceiverLocale(locale); /* save for next steps */
                  });
                  return assistedLocale;
              }
              return OptionalThing.ofNullable(null, () -> {
                  throw new SMailIllegalStateException("Not found the locale: " + postcard);
              });
          }
      }
docs: ------