window.addEvent('load', function(){
  new Element('span', {
    'id': 'back-to-top', 
    'styles': {'opacity': 0.85},
    'events': {'click': function() {
      if (window.location.hash) { window.location.hash = "page"; } 
      else { window.scrollTo(0, 0); }
    }}
  })
  .setText('back to top')
  .injectInside(document.body);
 
  window.addEvent('scroll', function() {
    var fn = window.getScrollTop() > (window.getHeight() * 0.8) ? "addClass" : "removeClass";
    $('back-to-top')[fn]('visible');
  });
});