[xsl] Grouping Based on the Number of Preceding Siblings

<root>
   <item id="someid001"/>
   <item id="someid002"/>
   <item id="someid003"/>
   ...
   <item id="someid999"/>
</root>

<root>
   <group>
       <item id="someid001"/>
          ...
       <item id="someid050"/>
   </group>
   <group>
       <item id="someid051"/>
       ...
       <item id="someid100"/>
   </group>
   ...
   <group>
       <item id="someid851"/>
       ...
       <item id="someid999"/>
   </group>
</root>

Thanks,
Jeff