code: def do_execute(self):
          
          data = self.input.payload
          cls = self.resolve_option("setup")
          if isinstance(cls, Classifier):
              cls = Classifier.make_copy(cls)
              evl = Evaluation(data)
              evl.discard_predictions = bool(self.resolve_option("discard_predictions"))
              evl.crossvalidate_model(
                  cls,
                  data,
                  int(self.resolve_option("folds")),
                  Random(int(self.resolve_option("seed"))),
                  self.resolve_option("output"))
              self._output.append(Token(evl))
          elif isinstance(cls, Clusterer):
              cls = Clusterer.make_copy(cls)
              evl = ClusterEvaluation()
              llh = evl.crossvalidate_model(
                  cls,
                  data,
                  int(self.resolve_option("folds")),
                  Random(int(self.resolve_option("seed"))))
              self._output.append(Token(llh))
          else:
              return "Unhandled class: " + classes.get_classname(cls)
          return None
docs: The actual execution of the actor.
  
          :return: None if successful, otherwise error message
          :rtype: str