chickadee » srfi-113 » set-adjoin

The set-adjoin procedure returns a newly allocated set that uses the same comparator as set and contains all the values of set, and in addition each element unless it is already equal (in the sense of the comparator) to one of the existing or newly added members. It is an error to add an element to set that does not return #t when passed to the type test procedure of the comparator.