[O] help advicing org-todo

[Luca Ferrari]

Hi all,
I'd like to advice the function org-todo to check in and check out the
clock automatically.
I've defined the org-todo-keywords so that they have a choice letter as follows:

(setq org-todo-keywords
      '( (sequence "TODO(t!)" "|" "DONE(d!)")
         (sequence "FEATURE(f!)" "BUG(b!)"  "IMPLEMENTING(i!)" "|"
"COMPLETED(c!)")
         ))

and I'd like to start the clock each time the "i" (implementing) is
chosen, stopping the clock each time the "c" (completed) is selected.
However I've a little problem doing the advice because the argument is
null and therefore I don't know which letter the user has pressed.
Anyone can provide a little help?

Thanks,
Luca