SYMMETRIC-DIFFERENCE-OF-SETS
============================

syntax: (symmetric-difference-of-sets
             <evaluates-to <set-of A (...)>>
             <evaluates-to <set-of B (...)>>
             [:TYPE <evaluates-to type-name>]
             [:TEST <lisp-type: function>])

        ==> #<set-of X (...)>

This function produces the symmetric difference of
two sets.

Warning: type checking may cause an error!

         Speed UP:
           (difference-of-sets
             (union-of-sets set1 set2 
                            :test test)
             (intersection-of-sets
                            set1 set2
                            :test test)
             :test test)


 
example: >(symmetric-difference-of-sets
                   *capital-cities* *cities*)
         #<set-of CITY (...)>
 
         >(set-to-list *)
         (#<CITY BONN> #<CITY DORTMUND>)