Creating index in Headers and Lines

Hi everyone,

I have the following XML.

<header>

     <key/>

     <dummy1/>

</header>

<line>

     <key/>

     <dummy2/>

</line>

I want to populate a counter/index in field key of header and line.

A header can have many lines.

In the XML file, there can be many headers.

the header and lines are connected via field key.

Example:

<header>

     <key>1</key>

     <dummy1>abcd</dummy1>

</header>

<line>

     <key>1</key>

     <dummy2>aaaa</dummy2>

</line>

<line>

     <key>1</key>

     <dummy2>bbbbb</dummy2>

</line>

<header>

     <key>2</key>

     <dummy1>abcd</dummy1>

</header>

<line>

     <key>2</key>

     <dummy2>eeee</dummy2>

</line>

<line>

     <key>2</key>

     <dummy2>rrrr</dummy2>

</line>

<line>

     <key>2</key>

     <dummy2>qqqq</dummy2>

</line>

I tried to use the INDEX or COUNT in Message Mapping but I did not succeed.

Can someone help ?

Thanks,

Regards.