code: def update_system_numbers(self):
          
          scn_035_fields = record_get_field_instances(self.record, '035')
          new_fields = []
          for field in scn_035_fields:
              subs = field_get_subfields(field)
              if '9' in subs:
                  if subs['9'][0].lower() == "cds" and subs.get('a'):
                      self.add_control_number("001", subs.get('a')[0])
                  if subs['9'][0].lower() in ["inspire", "spirestex", "inspiretex", "desy", "cds"]:
                      continue
              new_fields.append(field_get_subfield_instances(field))
          record_delete_fields(self.record, tag="035")
          for field in new_fields:
              record_add_field(self.record, tag="035", subfields=field)
docs: 035 Externals.