怎样让（chr 0）返回“\000”?

wfsyz

;这程序出错，因为(chr 0)返回空字符串，导致数值为0的字节丢失
(VL-LOAD-COM)
(defun tran1 (str1)
  (cond
    ((= str1 "0") (setq b1 0))
    ((= str1 "1") (setq b1 1))
    ((= str1 "2") (setq b1 2))
    ((= str1 "3") (setq b1 3))
    ((= str1 "4") (setq b1 4))
    ((= str1 "5") (setq b1 5))
    ((= str1 "6") (setq b1 6))
    ((= str1 "7") (setq b1 7))
    ((= str1 "8") (setq b1 8))
    ((= str1 "9") (setq b1 9))
    ((= str1 "a") (setq b1 10))
    ((= str1 "b") (setq b1 11))
    ((= str1 "c") (setq b1 12))
    ((= str1 "d") (setq b1 13))
    ((= str1 "e") (setq b1 14))
    ((= str1 "f") (setq b1 15))
    (t (setq b1 0))
  ) ;_ cond
  b1
) ;_ defun

(setq fi (open "d:/t2.txt" "R"))
(setq buffer (READ-LINE fi))
(close fi)
(setq n (/ (strlen buffer) 2))
(setq i 1)
(setq str2 "")
(repeat n
  (setq a1 (substr buffer i 1))
  (setq a2 (substr buffer (+ i 1) 1))
  (setq c1 (tran1 a1))
  (setq c2 (tran1 a2))
  (setq c3 (+ (* 16 c1) c2))
  (setq strc3 (chr c3))
  (setq str2 (strcat str2 strc3))
  (setq i (+ i 2))
) ;_ repeat
(setq fo (open "d:/t2.fas" "w"))
(princ str2 fo)
(close fo)