code: public IntervalCollection<T> plus(ChronoInterval<T> interval) {
  
          if (interval.isEmpty()) {
              return this;
          }
  
          List<ChronoInterval<T>> windows = new ArrayList<>(this.intervals);
          windows.add(this.adjust(interval));
          windows.sort(this.getComparator());
          return this.create(windows);
  
      }
docs: /*[deutsch]
  <p>F&uuml;gt das angegebene Intervall hinzu. </p>
  
  <p>Ein leeres Intervall wird ignoriert. </p>
  
  @param   interval    the new interval 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 interval is finite and has
  open start which cannot be adjusted to one with closed start
  @since   2.0