code: protected function calculateGoodLines(Model\DocumentInterface $document): Amount
      {
          $gross = new Amount($document->getCurrency());
  
          foreach ($document->getLinesByType(Model\DocumentLineTypes::TYPE_GOOD) as $line) {
              if (null !== $result = $this->calculateGoodLine($line)) {
                  $gross->merge($result);
              }
          }
  
          $gross->copyGrossToUnit();
  
          return $gross;
      }
docs: Calculates the good lines.
  
  @param Model\DocumentInterface $document
  
  @return Amount