Theorems about sbyte16 and IHS functions.
Theorem:
(defthm sbyte16p-of-logext-of-16 (sbyte16p (logext 16 x)))
Theorem:
(defthm logext-of-16-when-sbyte16p (implies (sbyte16p x) (equal (logext 16 x) x)))
Theorem:
(defthm logtail-of-16-when-sbyte16p (implies (sbyte16p x) (equal (logtail 16 x) (if (< x 0) -1 0))))