code: def correspondence(self):
          
          fields = 'surname initials organization country city_group'
          auth = namedtuple('Correspondence', fields)
          corr = self._head.get('correspondence')
          if corr is None:
              return None
          aff = corr.get('affiliation', {})
          try:
              org = aff['organization']
              if isinstance(org, dict):
                  try:
                      org = org['$']
                  except TypeError:  # Multiple names given
                      org = [d['$'] for d in org]
          except KeyError:
              org = None
          return auth(surname=corr.get('person', {}).get('ce:surname'),
                      initials=corr.get('person', {}).get('ce:initials'),
                      organization=org, country=aff.get('country'),
                      city_group=aff.get('city-group'))
docs: namedtuple representing the author to whom correspondence should
          be addressed, in the form
          (surname, initials, organization, country, city_group).