code: public function add($message)
      {
          if (is_string($message)) {
              $this->messages[] = new SimpleResponse($message);
          } elseif ($message instanceof ResponseInterface) {
              $this->messages[] = $message;
          } elseif ($message instanceof QuestionInterface) {
              $this->messages[] = $message;
          }
  
          return $this;
      }
docs: Add a message.
  
  @param string|Dialogflow\Action\Interfaces\ResponseInterface|Dialogflow\Action\Interfaces\QuestionInterface $message
  
  @return Conversation