docs: def form_field_definitions(self):
          """Hook optional_login extractor if necessary for form defaults.
          """
          schema = copy.deepcopy(form_field_definitions.user)
          uid, login = self._get_auth_attrs()
          if uid != login:
              field = schema.get(login, schema['default'])
              if field['chain'].find('*optional_login') == -1:
                  field['chain'] = '%s:%s' % (
                      '*optional_login', field['chain'])
                  if not field.get('custom'):
                      field['custom'] = dict()
                  field['custom']['optional_login'] = \
                      (['context.optional_login'], [], [], [], [])
              schema[login] = field
          return schema