Additional theorems about nibble.
Theorem:
(defthm nibblep-forward-unsigned-byte-p (implies (nibblep x) (unsigned-byte-p 4 x)) :rule-classes :forward-chaining)
Theorem:
(defthm unsigned-byte-p-rewrite-nibblep (equal (unsigned-byte-p 4 x) (nibblep x)))
Theorem:
(defthm natp-when-nibblep (implies (nibblep x) (natp x)) :rule-classes :compound-recognizer)