func (cli *OpsGenieAlertClient) AddNote(req alerts.AddNoteAlertRequest) (*alerts.AddNoteAlertResponse, error) {
	req.ApiKey = cli.apiKey
	// validate the mandatory parameters: apiKey, alertId/alias, note
	if req.ApiKey == "" {
		return nil, errors.New("ApiKey is a mandatory field and can not be empty.")
	}
	if req.Note == "" {
		return nil, errors.New("Note is a mandatory field and can not be empty")
	}
	if req.AlertId == "" && req.Alias == "" {
		return nil, errors.New("At least either Alert Id or Alias should be set in the request.")
	}
	if req.AlertId != "" && req.Alias != "" {
		return nil, errors.New("Either Alert Id or Alias should be set in the request not both.")
	}
	var resp *goreq.Response
	var err error
	for i := 0; i < cli.retries; i++ {
		resp, err = cli.buildRequest("POST", ADD_NOTE_ALERT_URL, req).Do()
		if err == nil {
			break
		}
		time.Sleep(TIME_SLEEP_BETWEEN_REQUESTS)
	}
	if err != nil {
		return nil, errors.New("Can not add note, unable to send the request.")
	}
	// check the returning HTTP status code
	httpStatusCode := resp.StatusCode
	if httpStatusCode >= 400 && httpStatusCode < 500 {
		return nil, errors.New(fmt.Sprintf("Client error %d returned", httpStatusCode))
	}
	if httpStatusCode >= 500 {
		return nil, errors.New(fmt.Sprintf("Server error %d returned", httpStatusCode))
	}
	var addNoteAlertResp alerts.AddNoteAlertResponse
	// check if the response can be unmarshalled
	if err = resp.Body.FromJsonTo(&addNoteAlertResp); err != nil {
		return nil, errors.New("Server response can not be parsed.")
	}
	return &addNoteAlertResp, nil
}