ELEMENT-OF-SET ============== syntax: (element-of-set <evaluates-to <set-of A (...)>> [:EXTRACT-FN <lisp-type: function>]) ==> <element> | NIL This function returns an element of the set, which is extracted by the extraction function. If there is not an extracted element, then NIL will be returned. example: >(list-to-set '(2 "house" 3 "window" 4 5)) #<set-of T (...)> >(element-of-set * :extract-fn #'stringp) "house"