code: def do_execute(self):
          
          if isinstance(self.input.payload, Instances):
              inst = None
              data = self.input.payload
          else:
              inst = self.input.payload
              data = inst.dataset
  
          retrain = False
          if (self._header is None) or (self._header.equal_headers(data) is not None) or (inst is None):
              retrain = True
              self._header = Instances.template_instances(data, 0)
  
          if retrain or (self._model is None):
              cls = self.resolve_option("setup")
              if isinstance(cls, Classifier):
                  self._model = Classifier.make_copy(cls)
              elif isinstance(cls, Clusterer):
                  self._model = Clusterer.make_copy(cls)
              elif isinstance(cls, Associator):
                  self._model = Associator.make_copy(cls)
              else:
                  return "Unhandled class: " + classes.get_classname(cls)
  
          if retrain:
              if inst is not None:
                  data = Instances.template_instances(data, 1)
                  data.add_instance(inst)
              if isinstance(self._model, Classifier):
                  self._model.build_classifier(data)
              elif isinstance(self._model, Clusterer):
                  self._model.build_clusterer(data)
              elif isinstance(self._model, Associator):
                  self._model.build_associations(data)
          else:
              if isinstance(self._model, Classifier):
                  self._model.update_classifier(inst)
              elif isinstance(self._model, Clusterer):
                  self._model.update_clusterer(inst)
              else:
                  return "Cannot train incrementally: " + classes.get_classname(self._model)
  
          cont = ModelContainer(model=self._model, header=self._header)
          self._output.append(Token(cont))
          return None
docs: The actual execution of the actor.
  
          :return: None if successful, otherwise error message
          :rtype: str