code: func Login(cookie string, secret []byte) string {
  	l, exp, err := Parse(cookie, secret)
  	if err != nil || exp.Before(time.Now()) {
  		return ""
  	}
  	return l
  }
docs: // Login returns a valid login extracted from the given cookie and verified
  // using the given secret key.  If verification fails or the cookie expired,
  // the function returns an empty string.