automatic continued numbering

Hello,

I'm trying to create a template for usecases. I would like to have automatic 
numbering for alternative courses.

eg.:
<usecase>
  <actor>bla</actor>
  <system><alternative>1</alternative>bla</system>
  <actor>bla</actor>
  <alternativecourses>
    <alternative>
      <system>ble</system>
      <actor>ble</actor>
    </alternative>
  </alternativecourses>
</usecase>

with output
1 bla
2	bla
3 bla
alternatives
  2a	ble
  3a ble

The automatic numbering for the standard usecase works, but I can't find how I 
get these alternatives working.
I tried to use keys, but because these alternatives are local for the usecase 
I couldn't find a way to make the match uniquely.


ps:  Is there a repository or something where there are collected a lot of 
extentions for docbook? Because I don't think that usecases are so seldom 
used that nobody will have created a template for it.

Thanks,

Wim Lemkens