code: func instanceLength(r *http.Response) (l int64, err error) {
  	switch r.StatusCode {
  	case http.StatusOK:
  		l, err = strconv.ParseInt(r.Header.Get("Content-Length"), 10, 64)
  		return
  	case http.StatusPartialContent:
  		cr, parseOk := httptoo.ParseBytesContentRange(r.Header.Get("Content-Range"))
  		l = cr.Length
  		if !parseOk {
  			err = errors.New("error parsing Content-Range")
  		}
  		return
  	default:
  		err = errors.New("unhandled status code")
  		return
  	}
  }
docs: // ok is false if the response just doesn't specify anything we handle.