Problems understanding functions.

Hi all

I must be reading the wrong documentation but i just can't get the following
function working.
I don't get an error but it appears as if the function goes in to a loop.
I could do this with PHP but would really like to get it right in plpgsql.

could someone please look at it and also add a 'plpgsql for dummies' URL

thanx Duncan.

drop function fun_power_ups();
create function fun_power_ups() RETURNS OPAQUE as '
begin
update power set ups = substring(power.phase from 1 for 1)
where old.power_id = new.power_id;
return new;
end;
' language 'plpgsql';

drop trigger tri_power_ups on power;
create trigger tri_power_ups after insert or update on power for each row
execute PROCEDURE fun_power_ups();

(this is all on one table.)