[docbook-apps] numbering parts and articles

Hi,

I need some help with part/article numbering. My book has the following structure:

<book>
  <part>
    <article/>
    <article/>
  </part>
  <part>
    <article/>

    <article/>
  </part>
</book>

I'm
 able control the numbering of part elements, but I cannot do it for article 
elements. I would like each article within a part element to contain the
 number of the current part and the number of the current article, 
separated by '.', i.e.

<book>        
  <part>            1. title
    <article/>        1.1 title
    <article/>        1.2 title
  </part>
  <part>            2. title

    <article/>        2.1 title
    <article/>        2.2 title
  </part>
</book>

Thanks for your help.
Robert