code: func RetryNotify(operation Operation, b Backoff, notify Notify) error {
  	var err error
  	var wait time.Duration
  	var retry bool
  	var n int
  
  	for {
  		if err = operation(); err == nil {
  			return nil
  		}
  
  		n++
  		wait, retry = b.Next(n)
  		if !retry {
  			return err
  		}
  
  		if notify != nil {
  			notify(err)
  		}
  
  		time.Sleep(wait)
  	}
  }
docs: // RetryNotify calls notify function with the error and wait duration
  // for each failed attempt before sleep.