Near alphabetic standard characters

near-chars.cl

(defun char-next (char)
  "Return the next alphabetic character, per CL standard."
  (assert (char-lessp char #\Z)
          (char) "No further characters in the standard alphabet.")
  (let* ((char-num (digit-char-p (char-upcase char) 36))
         (char-next (digit-char (1+ char-num) 36)))
    (if (lower-case-p char)
        (char-downcase char-next)
        char-next)))

(defun char-prev (char)
  "Return the next alphabetic character, per CL standard."
  (assert (char-greaterp char #\A)
          (char) "No previous characters in the standard alphabet.")
  (let* ((char-num (digit-char-p (char-upcase char) 36))
         (char-prev (digit-char (1- char-num) 36)))
    (if (lower-case-p char)
        (char-downcase char-next)
        char-next)))