code: def get_DOI(self):
          
          if self.dtd_name == 'JPTS':
              doi = self.root.xpath("./front/article-meta/article-id[@pub-id-type='doi']")
              if doi:
                  return doi[0].text
              log.warning('Unable to locate DOI string for this article')
              return None
          else:
              log.warning('Unable to locate DOI string for this article')
              return None
docs: This method defines how the Article tries to detect the DOI.
  
          It attempts to determine the article DOI string by DTD-appropriate
          inspection of the article metadata. This method should be made as
          flexible as necessary to properly collect the DOI for any XML
          publishing specification.
  
          Returns
          -------
          doi : str or None
              The full (publisher/article) DOI string for the article, or None on
              failure.