code: public D plus(D duration) {
  
          if (duration.isEmpty()) {
              return this.self();
          }
  
          long value = this.amount;
          return this.with(MathUtils.safeCast(value + duration.getAmount()));
  
      }
docs: /*[deutsch]
  <p>Liefert eine Kopie mit der addierten Dauer. </p>
  
  @param   duration    the duration to be added
  @return  result of addition as immutable copy
  @throws  ArithmeticException if numeric overflow occurs