function history_back(event) {
  event.stop();
  history.back();
}

Event.observe(window, 'load', function() {
  var links = $$('a');
  links.each(function (item) {
    if(item.rel=='external' || (item.protocol=='http:' && item.href.indexOf('http://' + window.location.host,0))==-1) {
      item.target = '_blank';
      item.rel='';
    }
  });
  var backs = $$('a[rel="back"]');
  if (backs) {
    backs.each(function(back) {back.observe('click', history_back.bindAsEventListener(back))});
  }
});