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

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

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

This function creates a new set with the difference 
of two sets. The new set has the element type of the 
first set.

Warning: type checking may cause an error!

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