SUBSET-OF-SET
=============

syntax: (subset-of-set <set-of type-A>
                       [:EXTRACT-FN <extract-fn>]
                       [:TEST test-fn]
                       [:TYPE <type-identifier>])

        ==> <set-of type-x>

This function produces a subset of the given set.

Warning: type checking may cause an error!

example: >(setf *capital-cities*
              (subset-of-set *cities*
              :EXTRACT-FN #'(lambda (a-element)
              (is-frame-of a-element 'capital-city))
              :TYPE 'capital-city))
         <set-of CAPITAL-CITY (...)>

         >(map-set *)
         (Paris Bruessel Berlin)