Less-than-or-equal test for characters
(char<= x y) is true if and only if the character code of
x is less than or equal to 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)))