code: private function throwIllegalOperationIfAdjustmentIsImmutable(ResourceEventInterface $event)
      {
          if ($event->getHard()) {
              return;
          }
  
          $adjustment = $this->getAdjustmentFromEvent($event);
  
          // Stop if adjustment is immutable.
          if ($adjustment->isImmutable()) {
              throw new IllegalOperationException('ekyna_commerce.sale.message.immutable_element');
          }
      }
docs: Throws an illegal operation exception if the adjustment is immutable.
  
  @param ResourceEventInterface $event
  
  @throws IllegalOperationException