code: def subject_areas(self):
          
          path = ['subject-areas', 'subject-area']
          area = namedtuple('Subjectarea', 'area abbreviation code')
          areas = [area(area=item['$'], code=item['@code'],
                        abbreviation=item['@abbrev'])
                   for item in chained_get(self._json, path, [])]
          return areas or None
docs: List of named tuples of subject areas in the form
          (area, abbreviation, code) of author's publication.