code: public Weekday getNextMatch(Weekday weekday) {
  
      if (contains(weekday)) {
        return weekday;
      } else if (getSet().isEmpty()) {
        return null;
      } else {
        Weekday result = weekday.getNext();
        while (!contains(result)) {
          result = weekday.getNext();
          if (result == weekday) {
            return null;
          }
        }
        return result;
      }
    }
docs: This method gets the next {@link Weekday} {@link #contains(Weekday) contained} in this {@link WeekdaySet} starting
  with the given {@code weekday}. If the given {@code weekday} is {@link #contains(Weekday) contained}, it will be
  returned. Otherwise the {@link Weekday#getNext() next} successor is determined.
  
  @param weekday is the {@link Weekday} to start with.
  @return the next {@link Weekday} starting with the given {@code weekday} or {@code null} if this {@link WeekdaySet}
  is {@link Set#isEmpty() empty}.