Changing variables



I have what is probably a very simple question.  I have a function 
defined in Maxima, say the following:

G(s) := 1/(3*s^2+2*s+1)

Now I would like to define a variable r such that

r^2 = 3*s^2

and then define G(s) in terms of this new variable r.  How does one go 
about achieving this in Maxima?

Many thanks in advance for any help.

-- 
--Moby

They that can give up essential liberty to obtain a little temporary safety deserve neither liberty nor safety.  -- Benjamin Franklin