;;; symmetric tree
;;; predicate, returns t when the right subtree is the mirror image of
;;; the left subtree (ignoring content)

(defun symmetric (tree)
  (or (null tree)
      (equal-structure (second tree) (revert (third tree)))
      )
  )

(defun revert (tree)
  (if (null tree)
      ()
    (list (first tree) (revert (third tree)) (revert (second tree)))
    )
  )

(defun equal-structure (t1 t2)
  (cond
   ((null t1) (null t2))
   ((null t2) nil)
   (t (and (equal-structure (second t1) (second t2))
	   (equal-structure (third  t1) (third  t2))
	   )
      )
   )
  )