code: public HijriCalendar plus(
          int amount,
          Unit unit
      ) {
  
          try {
              return unit.addTo(this, amount);
          } catch (IllegalArgumentException iae) {
              ArithmeticException ex = new ArithmeticException(iae.getMessage());
              ex.initCause(iae);
              throw ex;
          }
  
      }
docs: /*[deutsch]
  <p>Addiert die angegebenen Zeiteinheiten zu dieser Instanz. </p>
  
  @param   amount      amount to be added (maybe negative)
  @param   unit        calendrical unit
  @return  result of addition as changed copy, this instance remains unaffected
  @throws  ArithmeticException in case of numerical overflow
  @since   3.14/4.10