code: public static Weeks between(CalendarWeek w1, CalendarWeek w2) {
  
          PlainDate d1 = w1.at(Weekday.MONDAY);
          PlainDate d2 = w2.at(Weekday.MONDAY);
          return Weeks.between(d1, d2);
  
      }
docs: /*[deutsch]
  <p>Bestimmt die Differenz zwischen den angegebenen Kalenderwochen. </p>
  
  @param   w1  first calendar week
  @param   w2  second calendar week
  @return  difference in weeks