(defun mean-r (xs)
   (if (endp xs)
       (mv 0 0)
       (mv-let (m j)
               (mean-r (rest xs))
          (mv (+ (first xs) m) (+ j 1)))))

(defun mean (xs)
   (if (endp xs)
       0
       (mv-let (n d)
               (mean-r xs)
          (/ n d))))