docs: public synchronized GoogleCalendarService getCalendarService(String accountId) throws IOException {
          if (calendarService == null) {
              Credential credential = impl_getStoredCredential(accountId);
              if (credential == null) {
                  throw new UnsupportedOperationException("The account has not been authorized yet!");
              }
              calendarService = new GoogleCalendarService(impl_createService(credential));
          }
          return calendarService;
      }