Theorems about union$ in the std/lists library.
Theorem:
(defthm true-listp-of-union-equal (equal (true-listp (union-equal x y)) (true-listp y)))
Theorem:
(defthm true-listp-of-union-equal-type (implies (true-listp y) (true-listp (union-equal x y))) :rule-classes :type-prescription)