incrementing item numbers in docbook

Hi all,

I am creating a modular document with chapters and sections. Some of the
sections are labs.  I'm hoping to auto-number the labs, so I end up with
something like this:

Book
* Chapter 1
** Section 1
** Section 2: Lab 1.1
** Section 3
** Section 4: Lab 1.2
** Section 5: Lab 1.3
* Chapter 2
** Section 1
** Section 2: Lab 2.1
** Section 3: Lab 2.2

etc.

How do I autonumber the Lab titles?

Many thanks!

Paul