code: def texkeys(self, key, value):
      
      def _is_oai(id_, schema):
          return id_.startswith('oai:')
  
      def _is_desy(id_, schema):
          return id_ and schema in ('DESY',)
  
      def _is_texkey(id_, schema):
          return id_ and schema in ('INSPIRETeX', 'SPIRESTeX')
  
      texkeys = self.get('texkeys', [])
      external_system_identifiers = self.get('external_system_identifiers', [])
      _desy_bookkeeping = self.get('_desy_bookkeeping', [])
  
      values = force_list(value)
      for value in values:
          ids = force_list(value.get('a', ''))
          other_ids = force_list(value.get('z', ''))
          schema = force_single_element(value.get('9', ''))
  
          for id_ in ids:
              id_ = id_.strip()
              if not id_:
                  continue
  
              if _is_texkey(id_, schema):
                  texkeys.insert(0, id_)
              elif _is_oai(id_, schema):
                  continue  # XXX: ignored.
              elif _is_desy(id_, schema):
                  _desy_bookkeeping.append({'identifier': id_})
              else:
                  external_system_identifiers.insert(0, {
                      'schema': schema,
                      'value': id_,
                  })
  
          for id_ in other_ids:
              id_ = id_.strip()
              if not id_:
                  continue
  
              if _is_texkey(id_, schema):
                  texkeys.append(id_)
              elif _is_oai(id_, schema):
                  continue  # XXX: ignored.
              elif _is_desy(id_, schema):
                  _desy_bookkeeping.append({'identifier': id_})
              else:
                  external_system_identifiers.append({
                      'schema': schema,
                      'value': id_,
                  })
  
      self['external_system_identifiers'] = external_system_identifiers
      self['_desy_bookkeeping'] = _desy_bookkeeping
      return texkeys
docs: Populate the ``texkeys`` key.
  
      Also populates the ``external_system_identifiers`` and ``_desy_bookkeeping`` keys through side effects.