🚀 go-pugleaf

I have been using Epoch for a few months now, and this is the
first time I've had it crash consistently for me.  I've started
using some new Elisp stuff that a friend of mine wrote -
it's pretty simple stuff - but Epoch will crash every time
I use it.  The elisp code remembers window configurations in a
ring and lets you move around in it.  I'll usually save a few
configurations in it (3) and then move around.  Within a
few traversals of the ring, epoch dies with "Fatal error 11."

So the question is, what's wrong with this elisp code that
causes epoch to crash?  (I'm using epoch::version ->
"Epoch 4.0 Patchlevel 0".)

Thanks in advance...

--- window-ring.el ---

(defun put-at-end (elem list)
  (if (cdr list) (cons (car list) (put-at-end elem (cdr list)))
    (cons (car list) (list elem))))

(defun rotate-ring (ring)
  (cond ((not ring) ())         ; There is no ring to rotate
	((not (cdr ring)) ring) ; Ring has one member
	(t (put-at-end (car ring) (cdr ring)))))
    
(defun prepend-ring (elem ring)
  (cons elem ring))

(defun chop-ring (ring)
  (if (not ring) ()
    (cdr ring)))
    
(defvar window-config-ring ()) ; Start will a null ring

(defun chop-window-config-ring ()
  (interactive)
  (setq window-config-ring
	(chop-ring window-config-ring)))

(defun add-to-window-config-ring ()
  (interactive)
  (setq window-config-ring
	(prepend-ring (current-window-configuration) window-config-ring)))

(defun rotate-window-config-ring ()
  (interactive)
  (message "Moving thru' window ring...")
  (setq window-config-ring (rotate-ring window-config-ring))
  (set-window-configuration (car window-config-ring))
  (recenter nil))
  

(global-set-key "\C-c\C-a" 'add-to-window-config-ring)
(global-set-key "\C-t" 'rotate-window-config-ring)
(global-set-key "\C-c\C-q" 'chop-window-config-ring)

--- end of window-ring.el