code: func VerifyHTTPHeader(r *http.Request) error {
  	token := r.Header.Get(HeaderName)
  	if len(token) == 0 {
  		return trace.BadParameter("cannot retrieve CSRF token from HTTP header %q", HeaderName)
  	}
  
  	err := VerifyToken(token, r)
  	if err != nil {
  		return trace.Wrap(err)
  	}
  
  	return nil
  }
docs: // VerifyHTTPHeader checks if HTTP header value matches the cookie.