Visualize tree like data in Lisp

;;切换到工作目录
(cd "/home/reverland/Documents/lisp/")
;;;有向图
;;定义节点及描述
(defparameter *wizard-nodes* '((living-room (you are in the living-room.
                                             a wizard is snoring loudly on the couch.))
                               (garden (you are in a beautiful garden.
                                        w
                                       )
                                (attic (you are in the attic. there
                                        is a giant welding torch in the corner.)))))
;;定义节点与边
(defparameter *wizard-edges* '((living-room (garden west door)
                                (attic upstairs ladder))
                               (garden (living-room east door))
                               (attic (living-room downstairs ladder))))
  ;;转换名函数
(defun dot-name (exp)
 (substitute-if #\_ (complement #'alphanumericp) (prin1-to-string exp)))
  ;将非字母数字的字符转化为下划线 
  ;;定义标签长度
(defparameter *max-label-length* 30)
  ;;处理位置描述
(defun dot-label (exp)
 (if exp
  (let ((s (write-to-string exp :pretty nil))); :pretty防止换行或加入tab
   (if (> (length s) *max-label-length*)
    (concatenate 'string 
     (subseq s 0 (- *max-label-length* 3)) "...")
    ;如果标签长度比最长值长，将超长的替换成...
    s))
  ""));strange,why not nil?
  ;;定义节点和标签函数
(defun nodes->dot (nodes)
 (mapc (lambda (node);mapc不返回列表
        (fresh-line)
        (princ (dot-name (car node)))
        (princ "[label=\"")
        (princ (dot-label node))
        (princ "\"];"))
  nodes))
  ;;定义遍历edges中每个元素，再对每个元素进行遍历
  (defun edges->dot (edges)
   (mapc (lambda (node)
          (mapc (lambda (edge)
                 (fresh-line)
                 (princ (dot-name (car node)))
                 (princ "->")
                 (princ (dot-name (car edge)))
                 (princ "[label=\"")
                 (princ (dot-label (cdr edge)))
                 (princ "\"];"))
           (cdr node)))
    edges))
  ;;生成dot文件内容
(defun graph->dot (nodes edges)
 (princ "digraph{")
 (nodes->dot nodes)
 (edges->dot edges)
 (princ "}"))
  ;;接受一个trunk，得到trunk的输出而非函数的值
  ;;to keep this dot->png function as reusable as possible, the graph->dot
  ;;function isn’t called directly. Instead, we write dot->png to accept a thunk
(defun dot->png (fname thunk)
 (with-open-file (*standard-output*;类比与let
                  fname ;输入到fname文件
                  :direction :output;keyword symbol即本身,方向输出
                  :if-exists :supersede);如果存在覆盖
  (funcall thunk));nully function
 (ext:shell (concatenate 'string "dot -Tpng -O " fname)))
;it's an O actually!it's not zero!
;;最后一步
(defun graph->png (fname nodes edges)
 (dot->png fname
  (lambda ();relay funcion
   (graph->dot nodes edges))));which is the trunk,I am not clear.
  ;;最后执行
  (graph->png "wizard.dot" *wizard-nodes* *wizard-edges*)
  ;;;无方向图
(defun uedges->dot (edges)
 (maplist (lambda (lst);遍历剩下的元素
           (mapc (lambda (edge)
                  (unless (assoc (car edge) (cdr lst));除非以下不再出现这个位置
                   (fresh-line)
                   (princ (dot-name (caar lst)))
                   (princ "--")
                   (princ (dot-name (car edge)))
                   (princ "[label=\"")
                   (princ (dot-label (cdr edge)))
                   (princ "\"];")))
            (cdar lst)))
  edges))
(defun ugraph->dot (nodes edges)
 (princ "graph{")
 (nodes->dot nodes)
 (uedges->dot edges)
 (princ "}"))
(defun ugraph->png (fname nodes edges)
 (dot->png fname
  (lambda ()
   (ugraph->dot nodes edges))))
  ;;执行命令
  (ugraph->png "uwizard.dot" *wizard-nodes* *wizard-edges*)