var anchors = $$('#product_tabs a');

anchors.each(function(anchor) {
  anchor.box = anchor.href.sub(/^.*#/, '');
  Event.observe(anchor, 'click', function(event) {
    anchors.each(function(anchor) {
      $(anchor.parentNode).removeClassName('tab-active');
      $(anchor.box).removeClassName('collateral-active');
    });
    $(Event.element(event).parentNode).addClassName('tab-active');
    $(Event.element(event).box).addClassName('collateral-active');
    Event.stop(event);
  });
});