code: func (b BackoffPolicy) Duration(n int) time.Duration {
  	if n >= len(b.Millis) {
  		n = len(b.Millis) - 1
  	}
  
  	return time.Duration(jitter(b.Millis[n])) * time.Millisecond
  }
docs: // Duration returns the time duration of the n'th wait cycle in a
  // backoff policy. This is b.Millis[n], randomized to avoid thundering
  // herds.