Numbering

Hello !
Suppose i have an XML file like this :
<section level="1" title="foo">
   <section level="2" title="bar">
       <section level="3" title="foobar">
       </section>
   </section>
</section>

How can i number each section title ? :
1. Foo
   ...
1.1 Bar
   ...
1.1.1 Foobar
...
2.