apiClient.apiCall(arg)
  .flatMap(x -> x.result != null
             ? Observable.just(x.result)
             : x.error.isCaptcha()
               ? captchaDialog(x.error.captchaImgUrl)
                   // send the captcha
                   .flatMap(str -> apiClient.submitCaptcha(str))
                   // call the api method again
                   // this should be done recursively
                   // simplicity for the sake of blog post
                   .flatMap(success -> apiClient.apiCall(arg))
               : Observable.error(x.error))
  .subscribe(x -> // at this point we recovered from any captcha requests we encountered, and got our desired data,
             e -> // failure);