(let ((ring (make-ring 4)))
  (ring-insert "a")
  (ring-insert "b")
  (ring-insert "c")
  (ring-insert "d")
  (setq ring-var ring))

(defun rotate-fwd ()
  (let ((current-element (ring-remove ring-var)))
    (ring-insert ring-var current-element)))

(defun rotate-bwd ()
  (let ((current-element (ring-remove ring-var 0)))
    (ring-insert-at-beginning ring-var current-element)))