The source code

    /*!
     * Ext JS Library 3.2.1
     * Copyright(c) 2006-2010 Ext JS, Inc.
     * licensing@extjs.com
     * http://www.extjs.com/license
     */

    	if(Ext.isIE) {
            function fnCleanUp() {
                var p = Function.prototype;
                delete p.createSequence;
                delete p.defer;
                delete p.createDelegate;
                delete p.createCallback;
                delete p.createInterceptor;

                window.detachEvent("onunload", fnCleanUp);
            }
            window.attachEvent("onunload", fnCleanUp);
        }
    })();