(defun opentag (space key)
  (format nil "~a<~a>~%" space key))

(defun closetag (space key)
  (let ((key (string key)))
    ;; only print up to the first space
    (format nil "~a</~a>~%" space (subseq key 0 (position #\Space key)))))

(defun str (space str)
  (format nil "~a~a~%" space str))

(defun indent (space)
  "increase an indention level"
  (format nil "  ~a" space))

(defun strconcat (l)
  "concatenate a list of strings, return the new string"
  (reduce #'(lambda (a b) (concatenate 'string a b)) l))

(defun parse (l s)
  (strconcat (list
              (opentag s (car l))
              (strconcat (mapcar #'(lambda (i) (if (stringp i)
                                                   (str s i)
                                                 (parse i (indent s))))
                                 (cdr l)))
              (closetag s (car l)))))

(with-open-file (f "in.lsp") (princ (parse (read f) "")))