Writing overlays: examples available?

Hi all, I would like to learn how to write an overlay. I'm looking at overlay sources as available in the OpenLDAP package, but they appear to be too complex to me, particularly in not having clear comments/descriptions and, most of all, considering that I'm an occasional C programmer.

I ask if someone could send to me their overlay written for very simple goals or even for fun. Some code that I can easily follow and so have an idea of which hook use to do a specific thing. Commented source would be great! :-)

Thanks in advance Marco