(add-to-set-equal x lst) has a guard that lst is a true list.
Major Section: PROGRAMMING
For an object x and a true list lst, (add-to-set-equal x lst)
is the result of consing x on to the front of lst, unless
x is already a member of lst, in which case it equals lst.
(add-to-set-equal x lst) has a guard that lst is a true list.