code: static int invertWeekdayNum(ByDay weekdayNum, DayOfWeek dow0, int nDays) {
  		//how many are there of that week?
  		return countInPeriod(weekdayNum.getDay(), dow0, nDays) + weekdayNum.getNum() + 1;
  	}
docs: <p>
  Converts a relative week number (such as {@code -1SU}) to an absolute
  week number.
  </p>
  <p>
  For example, the week number {@code -1SU} refers to the last Sunday of
  either the month or year (depending on how this method was called). So if
  there are 5 Sundays in the given period, then given a week number of
  {@code -1SU}, this method would return 5. Similarly, {@code -2SU} would
  return 4.
  </p>
  @param weekdayNum the weekday number (must be a negative value, such as
  {@code -1SU})
  @param dow0 the day of the week of the first day of the week or month
  @param nDays the number of days in the month or year
  @return the absolute week number