code: def do_execute(self):
          
          if isinstance(self.input.payload, Instances):
              inst = None
              data = self.input.payload
          elif isinstance(self.input.payload, Instance):
              inst = self.input.payload
              data = inst.dataset
  
          index = str(self.resolve_option("index"))
          unset = bool(self.resolve_option("unset"))
          if unset:
              data.no_class()
          else:
              if index == "first":
                  data.class_is_first()
              elif index == "last":
                  data.class_is_last()
              else:
                  data.class_index = int(index) - 1
  
          if inst is None:
              self._output.append(Token(data))
          else:
              self._output.append(Token(inst))
  
          return None
docs: The actual execution of the actor.
  
          :return: None if successful, otherwise error message
          :rtype: str