code: def sync(self, old_token=None):
          
          # 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()
docs: 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.