Illustrator Script: How can I duplicate a layer ( with sublayers ) with javascript code ?

Thanks for your answer.

1. #target illustrator 
2.  
3. var docRef = app.activeDocument; 
4.  
5. var ln = 'Layer 1'; 
6.  
7. var ol = docRef.layers.getByName(ln); 
8.  
9. var nl = docRef.layers.add(); 
10.  
11. nl.name = ln+' Copy'; 
12.  
13. for (var a = ol.pageItems.length-1; a >= 0; a--) { 
14.  
15.     ol.pageItems.duplicate(nl, ElementPlacement.PLACEATBEGINNING); 
16.  
17. } 

The problem with this code is, that it can't duplicate also the sublayers, and the sublayer elements.

Sure I can write a recursive function, but I think there should exist a duplicate function somewhere ( for Photoshop script exist ! ) or a small trick, but I can't find it.

Thanks