how to group nodes by id?

 hi
 does anybody know how to convert the following XML:
 
 <elems>
 <elem id=1>
  <elem id=1/>
 </elem>
 <elem id=1>
  <elem id=2/>
 </elem>
 <elem id=2>
  <elem id=1/>
 </elem>
 <elem id=2>
  <elem id=2/>
 </elem>
 </elems>
 
 to the form
 
 <elems>
 <elem id=1>
  <elem id=1/>
  <elem id=2/>
 </elem>
 <elem id=2>
  <elem id=1/>
  <elem id=2/>
 </elem>
 </elems>
 
 thanks Artur
 

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