All you need is nth and setf:

emacs -Q, then evaluate the following:

(defun add-to-nth (x n ys)
  (when ys (setf (nth n ys) (+ x (nth n ys)))))

(setq foobar  '(1 2 3 4 5))
(add-to-nth 42 1 foobar)

C-h v foobar  ; =&gt;  (1 44 3 4 5)


lisp function that adds x to the nth item of a list
If a player wants to do something a little out of the ordinary such as punching down a wall with his bare fists, what should I do? Do I make a DC for it? Do I make it so that no matter how much he rolls, he won't destroy the wall? Should I simply tell him that it won't work?
