Theorems about sbyte8 and IHS functions.
Theorem:
(defthm sbyte8p-of-logext-of-8 (sbyte8p (logext 8 x)))
Theorem:
(defthm logext-of-8-when-sbyte8p (implies (sbyte8p x) (equal (logext 8 x) x)))
Theorem:
(defthm logtail-of-8-when-sbyte8p (implies (sbyte8p x) (equal (logtail 8 x) (if (< x 0) -1 0))))