average of a tree

gistfile1.txt

(defun average (list)
  (/ (tree-reduce #'+ list)
     (tree-reduce #'+ list :key (constantly 1))))

(defun tree-reduce (fn tree &key (key #'identity))
  (reduce fn tree
          :key (lambda (item)
                 (if (consp item)
                     (tree-reduce fn item :key key)
                   (funcall key item)))))