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

syntax: (is-subset-of-set <set-of type-A> 
                          <set-of type-B>
                          [:TEST test-fn])

        ==> <set-of type-A> | NIL

This function checks, whether the set of type A is 
part of the set of type B.

example: >(is-subset-of-set *capital-cities* 
                            *cities*)
         #<set-of T (...)>
      
         >(is-subset-of-set *cities* 
                            *capital-cities*)
         NIL