Some theorems about the library functions str::hex-digit-char-value and str::hex-digit-to-char.
Theorem:
(defthm str::hex-digit-char-value-of-hex-digit-to-char (implies (integer-range-p 0 16 n) (equal (str::hex-digit-char-value (str::hex-digit-to-char n)) n)))
Theorem:
(defthm str::hex-digit-to-char-of-hex-digit-char-value (implies (str::hex-digit-char-p char) (equal (str::hex-digit-to-char (str::hex-digit-char-value char)) (str::upcase-char char))))