Less-than test for characters
(char< x y) is true if and only if the character code of
x is less than that of y. See char-code.
The guard for char< specifies that its arguments are characters.
Char< is a Common Lisp function. See any Common Lisp documentation
for more information.
(defun char< (x y)
(declare (xargs :guard (and (characterp x) (characterp y))))
(< (char-code x) (char-code y)))