code: def object_id_doi(tag, parent_tag_name=None):
      
      doi = None
      object_id = None
      object_ids = raw_parser.object_id(tag, "doi")
      if object_ids:
          object_id = first([id_ for id_ in object_ids])
      if parent_tag_name and object_id and object_id.parent.name != parent_tag_name:
          object_id = None
      if object_id:
          doi = node_contents_str(object_id)
      return doi
docs: DOI in an object-id tag found inside the tag