code: public I collapse() {
  
          if (this.start.isInfinite()) {
              throw new IllegalStateException(
                  "An interval with infinite past cannot be collapsed.");
          }
  
          T t = this.getClosedFiniteStart();
          Boundary<T> s = Boundary.ofClosed(t);
          Boundary<T> e = Boundary.ofOpen(t);
          return this.getFactory().between(s, e);
  
      }
docs: /*[deutsch]
  <p>Wandelt dieses Intervall in ein leeres Intervall mit dem gleichen
  Startanker um. </p>
  
  @return  new empty interval with same start (anchor always inclusive)
  @throws  IllegalStateException if the start is infinite
  @since   2.0