trigger function in plpgsql (newbie)

Hello. I'm  a total newbie when it comes to plpgsql and trigger functions, but
here is my code that does not work

create table T1 (
    ID    integer primary key,
    v1    real );

create table T2 (
    RID    integer references  T1,
    when    date,
    v2    real,
    v1timesv2    real );

I whish to do the following:
Whenever I insert something into T2, I insert when and v2, I also wish to
calculate the value v1timesv2 and store it in the table. (the value of v1
changes over time). I wish to do this as a trigger function. I really don't
know how to do this with a plpgsql function, here is a futile attempt.

create function func_cal() RETURNS OPAQUE AS '
   BEGIN
      NEW.v1timesv2 := NEW.v2 * (select T1.v1 from T1 WHERE NEW.RID=T1.ID);
      RETURN NEW;
   END; '
LANGUAGE 'plpgsql';

create trigger trigger_happy AFTER INSERT ON  T2
    for each ROW EXECUTE PROCEDURE func_cal();

I guess you laugh at this, but I have not found any really good documentation
about this (or am I just stupid?) so please tell me what I do wrong.

By the way, in the name you write in the create trigger statement, when I
look at the man page for create_trigger it says
"name   The name of an existing trigger."
Eh... I thought I was creating a NEW trigger... any comments on this?

TIA,
Gunnar.