Index of upper level in PI Graphical mapping

Hello experts,

I have an incoming structure like this:

<source>

     <item>

          <subitem>

          </subitem>

          <subitem>

          </subitem>

     </item>

     <item>

          <subitem>

          </subitem>

     </item>

     <item>

     </item>

</source>

the target structure is:

<target>

     <item>

          <subitem>

               <item_index>  1</item_index>

               <subitem_index>  1</subitem_index>

          </subitem>

          <subitem>

               <item_index> 1</item_index>

               <subitem_index> 2  </subitem_index>

          </subitem>

     </item>

     <item>

          <subitem>

               <item_index>  2</item_index>

               <subitem_index>  1</subitem_index>

          </subitem>

     </item>

     <item>

     </item>

</target>

In other word, I need to get an index of Item node in the context of SubItem.

How can I do this using graphical mapping tool's functions?

I am able to get SubItem index using  Index[1,1] and SplitByValue functions, but for Item in the context of SubItem it doesnt work, as any Item may have different numbers of SubItems.

Any help is appreciated.

Thanks!