nesting-print.lisp

(defun make-nesting-print-fn (up-pre down-pre &optional (*standard-output* *standard-output*))
  (let ((counter 0))
    (list
     ;;count-up
     (lambda (obj)
       (incf counter)
       (dotimes (i counter)
	 (format t "~A" up-pre))
       (format t "~A~%" obj)
       obj)
     ;;count-down
     (lambda (obj)
       (dotimes (i counter)
	 (format t "~A" down-pre))
       (format t "~A~%" obj)
       (unless (zerop counter)
	 (decf counter))
       obj))))
(destructuring-bind (up down)
    (make-nesting-print-fn ">" "<")
  (setf (symbol-function 'nesting-print-up) up
	(symbol-function 'nesting-print-down) down))