defining a setter function with gv.el

[Ivan Kanis <ivan.kanis@googlemail.com>]

Stefan,

I am very new to setf. I would like to write a function that provides a
place for setf to work on an alist. It would be wonderful if the
function also worked as a getter.

(setq foo '((oak . tree)))

(defun my-function (&optional value)
  (if value
      (?)
    (cdr (assoc 'oak tree))))

(my-function) => tree

(my-function 'shrub)

foo => '((oak . shrub))

gv.el seems to have what I need to implement the setf portion. It
doesn't provide example and I couldn't find doc in the elisp info.

Could you, pleas, help me?

Take care,
-- 
Ivan Kanis
http://ivan.kanis.fr

Success is counted sweetest
By those who ne'er succeed.
To comprehend a nectar
Requires sorest need.
    -- Emily Dickinson