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What is the consequence of this bit of javascript?

I was browsing the source code of JQuery UI. I saw this line at the beginning of the js file:

;jQuery.ui || (function($) {

what does it do?

(more from the jquery.ui.core.js)

/*!
 * jQuery UI 1.8
 *
 * Copyright (c) 2010 AUTHORS.txt (http://jqueryui.com/about)
 * Dual licensed under the MIT (MIT-LICENSE.txt)
 * and GPL (GPL-LICENSE.txt) licenses.
 *
 * http://docs.jquery.com/UI
 */
;jQuery.ui || (function($) {
.ui = {
    version: "1.8",

    // $.ui.plugin is deprecated.  Use the proxy pattern instead.
    plugin: {
...

Answer 1

Breaking it down:

// make sure that any previous statements are properly closed
// this is handy when concatenating files, for example
; 
// Call the jQuery.ui object, or, if it does not exist, create it
jQuery.ui || (function($) {