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"