Another Plpgsql trigger example - summary table

I have always thought that an example of how to maintain a summary table
via triggers would be nice... but until the other day, had not attempted
to do one, so contributing a simplified version seemed like a good thing
to do.

I have made the example pretty much self contained, which unfortunately
means it is longish. This made me wonder about its placement (i.e in
plpgsql examples). It could go in a 'Data warehousing' chapter - if we
had one....

Any suggestions welcome.

best wishes

Mark

P.s : use is made of a schema from Ralph Kimball's "The Data Warehouse
Toolkit" - I mailed him to check it was ok (and it was, in fact he was
quite pleased)