def do_execute(self):
        """
        The actual execution of the actor.
        
        :return: None if successful, otherwise error message
        :rtype: str
        """
        for s in self.resolve_option("strings"):
            self._output.append(Token(s))
        return None
The actual execution of the actor.
        
        :return: None if successful, otherwise error message
        :rtype: str