code: public IntervalCollection<T> plus(IntervalCollection<T> other) {
  
          if (this == other) {
              return this;
          }
  
          return this.plus(other.getIntervals());
  
      }
docs: /*[deutsch]
  <p>&Auml;quivalent zu {@code plus(other.getIntervals())}. </p>
  
  @param   other       another interval collection whose intervals are to be added to this instance
  @return  new interval collection containing the intervals of this instance and the argument
  @throws  IllegalArgumentException if given collection contains a finite
  interval with open start which cannot be adjusted to one with closed start
  @since   3.7/4.5