code: protected function getSubjectAndRepository($subjectOrRelative)
      {
          if ($subjectOrRelative instanceof SubjectRelativeInterface) {
              $subject = $this->subjectHelper->resolve($subjectOrRelative);
          } elseif ($subjectOrRelative instanceof StockSubjectInterface) {
              $subject = $subjectOrRelative;
          } else {
              throw new InvalidArgumentException(sprintf(
                  "Expected instance of '%s' or '%s'.",
                  SubjectRelativeInterface::class,
                  StockSubjectInterface::class
              ));
          }
  
          return [$subject, $this->getRepositoryBySubject($subject)];
      }
docs: Returns the subject and his stock unit repository.
  
  @param StockSubjectInterface|SubjectRelativeInterface $subjectOrRelative
  
  @return array(StockSubjectInterface, StockUnitRepositoryInterface)