}
  }

  /** See https://api.slack.com/rtm. */
  public WebSocket rtm(HttpUrl url, WebSocketListener listener) {
    return httpClient.newWebSocket(new Request.Builder()
        .url(url)
        .build(), listener);
  }

  static final class SlackJsonAdapters {
    @ToJson String urlToJson(HttpUrl httpUrl) {
      return httpUrl.toString();
    }