CL: Check which CODE-CHARs do not yield characters

char-check.lisp

(defun char-check ()
  "Print all codes for which (code-char a) returns nil and return their number.
  
If this returns nonzero, there is a hole in the code-chars,
so be careful.
If this returns 0, the implementation is effectively
NIL-proof in this case.
NIL-bearing so far:
CCL 1.11-r16635 on Win32, beginning at 55295. 2048 codes (D800-DFFF) under 1114112 have a (code-char code) -> nil.
NIL-proof so far:
SBCL 1.2.16 on Win32
ECL 16.0 on Win32
ABCL 1.4.0-dev-svn-14845M on Java 1.7
CLISP 2.49 on Linux
JSCL 0.3.0 on Chrome"
  (let ((x 0))
    (dotimes (a char-code-limit)
      (when (null (code-char a))
        (format t "~X " a)
        (incf x)))
    x))