code: public IntervalCollection<T> plus(Collection<? extends ChronoInterval<T>> intervals) {
  
          if (intervals.isEmpty()) {
              return this;
          }
  
          List<ChronoInterval<T>> windows = new ArrayList<>(this.intervals);
  
          for (ChronoInterval<T> i : intervals) {
              if (!i.isEmpty()) {
                  windows.add(this.adjust(i));
              }
          }
  
          windows.sort(this.getComparator());
          return this.create(windows);
  
      }
docs: /*[deutsch]
  <p>F&uuml;gt die angegebenen Intervalle hinzu. </p>
  
  <p>Leere Intervalle werden ignoriert. </p>
  
  @param   intervals       the new intervals to be added
  @return  new IntervalCollection-instance containing a sum of
  the own intervals and the given one while this instance remains unaffected
  @throws  IllegalArgumentException if given list contains a finite
  interval with open start which cannot be adjusted to one with closed start
  @since   3.35/4.30