def do_execute(self):
        """
        The actual execution of the actor.

        :return: None if successful, otherwise error message
        :rtype: str
        """
        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
The actual execution of the actor.

        :return: None if successful, otherwise error message
        :rtype: str