// retrieves url with HEAD
static bool urlExists(S url) {
  url = absoluteURL(url);
  HttpURLConnection conn = null;
  InputStream in = null;
  try {
    conn = (HttpURLConnection) new URL(url).openConnection();
    conn.setRequestMethod("HEAD");
    conn.setRequestProperty("X-No-Cookies", "1");
    in = conn.getInputStream();
    true;
  } catch (IOException e) {
    false;
  } finally {
    closeURLConnection(conn, in);
  }
}