Event.observe(document, 'dom:loaded', function() {
  var referrer = document.referrer;
  if (referrer == '') {
    return;
  }
  var src = 'track_keyword.gif?cf_referer=' + encodeURIComponent(referrer);
  document.body.appendChild(new Element('img', {'src': src, 'style': 'display: none;'}));
});