code: protected function assertItemAdjustmentClass(Common\AdjustmentInterface $adjustment)
      {
          if (!$adjustment instanceof Model\CartItemAdjustmentInterface) {
              throw new InvalidArgumentException("Expected instance of " . Model\CartItemAdjustmentInterface::class);
          }
      }
docs: Asserts that the given adjustment is an instance of the expected class.
  
  @param Common\AdjustmentInterface $adjustment