code: def update_system_numbers(self):
          
          scn_035_fields = record_get_field_instances(self.record, '035')
          forbidden_values = ["cercer",
                              "inspire",
                              "xx",
                              "cern annual report",
                              "cmscms",
                              "wai01"]
          for field in scn_035_fields:
              subs = field_get_subfields(field)
              if '9' in subs:
                  if 'a' not in subs:
                      continue
                  for sub in subs['9']:
                      if sub.lower() in forbidden_values:
                          break
                  else:
                      # No forbidden values (We did not "break")
                      suffixes = [s.lower() for s in subs['9']]
                      if 'spires' in suffixes:
                          new_subs = [('a', 'SPIRES-%s' % subs['a'][0])]
                          record_add_field(
                              self.record, '970', subfields=new_subs)
                          continue
              if 'a' in subs:
                  for sub in subs['a']:
                      if sub.lower() in forbidden_values:
                          record_delete_field(self.record, tag="035",
                                              field_position_global=field[4])
docs: 035 Externals.