Question on example found at src/test of libpqxx 3.0

Hello list,

I've started using libpqxx some days ago and had no problem doing the basic
stuff so far... Right now i'm trying to code a simple test program to do
something when it receives a notification from the db. I was looking into
examples provided in the src/test directory of my libpqxx 3.0 installation,
specifically test004.cxx file. There i can see a class named TestListener
wich inherits from class notify_listener. Since i couldn't find the latter
class on the documentation, i assume it doesn't exist and it's just for
demo. Am i correct assuming that? If so, what would this class be? A
trigger? A transactor?
Any directions regarding better understanding of notify/listen would be
appreciated.

Thanx!
--
Matheus Araújo Aguiar
Computer Scientist
matheus(dot)pit(at)gmail(dot)com