code: func (p *LogPrefix) IsRetry(c context.Context, nonce []byte) bool {
  	if len(nonce) != types.OpNonceLength {
  		logging.Infof(c, "user provided invalid nonce length (%d)", len(nonce))
  		return false
  	}
  	if len(p.OpNonce) == 0 {
  		logging.Infof(c, "prefix %q has no associated nonce", p.Prefix)
  		return false
  	}
  	if clock.Now(c).After(p.Created.Add(RegistrationNonceTimeout)) {
  		logging.Infof(c, "prefix %q has expired nonce", p.Prefix)
  		return false
  	}
  	return subtle.ConstantTimeCompare(p.OpNonce, nonce) == 1
  }
docs: // IsRetry checks to see if this LogPrefix is still in the OpNonce
  // window, and if nonce matches the one in this LogPrefix.