Some theorems about the built-in function add-to-set.
Theorem:
(defthm true-listp-of-add-to-set-equal (equal (true-listp (add-to-set-equal a x)) (true-listp x)))
Theorem:
(defthm true-listp-of-add-to-set-equal-type (implies (true-listp x) (true-listp (add-to-set-equal a x))) :rule-classes :type-prescription)