add an object to a list
Major Section:  PROGRAMMING

For an object x and a true list lst, (add-to-set-eql 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-eql x lst) has a guard that lst is a true list and moreover, either x is eqlablep or lst is an eqlable-listp.