UNION-OF-SETS
=============

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

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

This function produces the union of two sets.

Warning: type checking may cause an error!

example: >(union-of-sets *cities* *capital-cities*)
         #<set-of CITY (...)>

         >(set-to-list *)
         (#<CITY BONN> #<CITY DORTMUND> #<CAPITAL-
              CITY PARIS> #<CAPITAL-CITY BRUESSEL>
                 #<CAPITAL-CITY BERLIN>)