docs: def sync(self, old_token=None):
          """Get the current sync token and changed items for synchronization.
  
          ``old_token`` an old sync token which is used as the base of the
          delta update. If sync token is missing, all items are returned.
          ValueError is raised for invalid or old tokens.
          """
          # FIXME: Actually implement
          token = "http://radicale.org/ns/sync/%s" % self.etag.strip("\"")
          if old_token:
              raise ValueError("Sync token are not supported (you can ignore this warning)")
          return token, self.list()