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<window xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">

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  <tree id="tree" flex="1" >
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          <treecell label="PHP"   />
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function addItem(title, path)
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var current = 0;
var items = [
  ['Java', 'programming/java'],
 ...............................................и тд
];
function addSomething()
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document.getElementsByTagName('tree')[x].getElementsByTagName('treeitem')[x].parentNode.removeChild(elem[0]);