Mootools 1.2 $extend

Question

my code index.php

var MyObject ={
  method1:function(){...},
  method2:function(){...},
  method3:$extend
};

MyObject .extend(MyObject ,Events.prototype);
MyObject .extend(MyObject ,Options.prototype);
MyObject .extend(MyObject ,Chain.prototype);

windown.onload = function()
{
   MyObject .method1();
}

at first load , no problem.

Then i click a link call again this page by modal joomla and i get problems

1.$extend is underfined

2.MyObject .method1 is underfined ( Object is underfined )

why ? Object allway in top , why not can define Object ?