code: public IntervalCollection<T> withIntersection() {
  
          int len = this.intervals.size();
  
          if (len < 2) {
              return this;
          } else {
              return this.create(this.intersect(this.intervals));
          }
  
      }
docs: /*[deutsch]
  <p>Ermittelt die Schnittmenge aller enthaltenen Intervalle. </p>
  
  <p>Hinweis: Diese Instanz bleibt unver&auml;ndert, weil die Klasse
  <i>immutable</i> (unver&auml;nderlich) ist. </p>
  
  <p><img src="doc-files/withIntersection.jpg" alt="withIntersection"></p>
  
  @return  new interval collection containing the intersection interval,
  maybe empty (if there is no intersection)
  @since   2.0