"Back to top" with mootools

Can anyone give me a similar script for mootools. I'm using jquery and I need a similar function like this to use with mootools:

$(document).ready(function(){
   if( $(".page")开发者_如何学JAVA.height() < $(window).height() ) {
       $(".backtop").hide();
   }
   else {
       $(".backtop").show();
   }
});

What I want is a "back to top" button that only appears when the div.page is bigger than the screen window.

$$('a.backtop').setStyle('display',
    document.getElement('div.page').getSize().y > window.getSize().y ? 'block' : 'none'
);

note sometimes on old ie you might have to wrap $(window) or $(document).

http://jsfiddle.net/wD6RY/

window.addEvent('domready', function() {
   ($('div.page')> window.getSize().y) ? $('div.backtop').fade(1) : $('div.backtop').fade(0)  
});