One of feat-little-endianp and feat-big-endianp always holds.
Theorem:
(defthm feat-little-or-big-endianp (or (feat-little-endianp feat) (feat-big-endianp feat)) :rule-classes (:rewrite (:forward-chaining :trigger-terms ((feat-little-endianp feat) (feat-big-endianp feat)))))