Setting an element in a nested list structure

nset.el

(defun nset-element-at (path ast new-element)
  (if (= 0 (length path))
      (setf ast new-element)
    (let ((place-fn)
          (path (reverse path)))
      (progn
        (dotimes (current-index (length path) nil)
          (setq place-fn
                (if (= current-index 0)
                    (list 'nth (nth current-index path) 'ast)
                  (list 'nth (nth current-index path) place-fn))))
        (eval `(setf ,place-fn new-element))))))

> (setq *x* '(1 (2 3 4 (5 (6 7 (8 9))))))
(1
 (2 3 4
    (5
     (6 7
        (8 9)))))
> (nset-element-at '(1 2 1 3 1) *x* 'x)
x
> *x*
(1
 (2 3 4
    (5
     (6 7
        (8 x)))))