code: public D plus(int amount) {
  
          if (amount == 0) {
              return this.self();
          }
  
          long value = this.amount;
          return this.with(MathUtils.safeCast(value + amount));
  
      }
docs: /*[deutsch]
  <p>Liefert eine Kopie mit dem addierten Betrag. </p>
  
  @param   amount      the amount to be added
  @return  result of addition as immutable copy
  @throws  ArithmeticException if numeric overflow occurs