code: def contributor_group(self):
          
          items = listify(chained_get(self._head, ['source', 'contributor-group'], []))
          out = []
          fields = 'given_name initials surname indexed_name role'
          pers = namedtuple('Contributor', fields)
          for item in items:
              entry = item.get('contributor', {})
              new = pers(indexed_name=entry.get('ce:indexed-name'),
                  role=entry.get('@role'), surname=entry.get('ce:surname'),
                  given_name=entry.get('ce:given-name'),
                  initials=entry.get('ce:initials'))
              out.append(new)
          return out or None
docs: List of namedtuples representing contributors compiled by Scopus,
          in the form (given_name, initials, surname, indexed_name, role).