code: public MultiDayOfTheWeek compress() {
  
          if (multipleDayOfTheWeek.size() == 1) {
              return this;
          }
  
          DayOfTheWeek start = null;
          final StringBuilder sb = new StringBuilder();
          for (int i = 0; i < multipleDayOfTheWeek.size(); i++) {
              final DayOfTheWeek current = multipleDayOfTheWeek.get(i);
              if (start == null) {
                  if (i > 0) {
                      sb.append("/");
                  }
                  start = current;
                  sb.append(start);
              } else {
                  if (i == multipleDayOfTheWeek.size() - 1) {
                      sb.append(separator(start, current));
                      sb.append(current);
                  } else {
                      if (multipleDayOfTheWeek.get(i + 1).previous() != current) {
                          sb.append(separator(start, current));
                          sb.append(current);
                          start = null;
                      }
                  }
              }
          }
          return new MultiDayOfTheWeek(sb.toString());
  
      }
docs: Returns a compressed version of this instance. Compressed means, that for example 'Mo/Tue/Wed/Thu/Fri' will be returned as 'Mon-Fri'.
  
  @return Shortened version.