code: public CalendarDays plus(CalendarDays other) {
  
          return CalendarDays.of(Math.addExact(this.days, other.days));
  
      }
docs: /*[deutsch]
  <p>Liefert die Summe der Kalendertage dieser Instanz und des angegebenen Arguments. </p>
  
  @param   other   calendar days to be added
  @return  sum of calendar days
  @since   3.4/4.3