code: def external_system_identifiers(endpoint):
      
      @utils.flatten
      @utils.for_each_value
      def _external_system_identifiers(self, key, value):
          new_recid = maybe_int(value.get('d'))
          if new_recid:
              self['new_record'] = get_record_ref(new_recid, endpoint)
  
          return [
              {
                  'schema': 'SPIRES',
                  'value': ext_sys_id,
              } for ext_sys_id in force_list(value.get('a'))
          ]
  
      return _external_system_identifiers
docs: Populate the ``external_system_identifiers`` key.
  
      Also populates the ``new_record`` key through side effects.