XSLT transformation to xml, grouping by key

Question

I have problem with write xsl to transform my xml to raport version. It looks like that:

    
        Jo         


        

Answer 1

This transformation:

when applied on the provided XML document:

    
        John
        Tom
    
    
        

            1001
            
        
        

            1002
            
        
        

            1001
            
        
    

produces the wanted, correct result:

   
      John
      
         
Article1
         
Article3
      
   
   
      Tom
      
         
Article2
      
   

Notes:

1. Use/overriding of the identity rule.

2. All articles with the same authorId are selected using keys. This is significantly more efficient in case of many authors with many articles.