code: func checkResults(gcmResults []Result, recipients []string, resultsState multicastResultsState) (doRetry bool, toRetry []string, err error) {
  	doRetry = false
  	toRetry = []string{}
  	for i := 0; i < len(gcmResults); i++ {
  		result := gcmResults[i]
  		regId := recipients[i]
  		resultsState[regId] = &result
  		if result.Error != "" {
  			if retryableErrors[result.Error] {
  				toRetry = append(toRetry, regId)
  				if doRetry == false {
  					doRetry = true
  				}
  			}
  		}
  	}
  	return doRetry, toRetry, nil
  }
docs: // For a multicast send, determines which errors can be retried.