More precision about generating functions (for people who don't know)

Hi,

I sent a couple of hours ago a message concerning my tiny package:
  http://tbaruchel.free.fr/ggf.mac
Two things:
  a) if people alrady had a look at it, please download it again,
     since I noticed a stupid loop could be replaced by a simple
     sum() function :-)
  b) if people wonder about generating function, let me explain
     a very simple usage for them :
     if you suspect a sequence 1, 1, 2, 3, 5, 8, ...
     is made from a linear recurrence relation, the denominator
     of the generating function (in my example: 1 - x - x^2 )
     should have in its coefficient the exact coefficients of
     this linear recurrence relation.

     I AM CONCERNED with speed or efficiency benchmarks if you
     already have your own functions for performing that task.
     I think my function should be very efficient and quick,
     but if you can compare it with some others, I will be very
     interested.

Regards,

-- 
Thomas Baruchel