IS-EMPTY-SET
============

syntax: (is-empty-set <evaluates-to set>)

        ==> T | NIL

This function checks, whether the given set is an 
empty set.

Warning: length works on vectors AND lists!

example: >(difference-of-set *capital-cities* 
                             *cities*)
         #<set-of T (...)>

         >(is-empty-set *)
         T    ; i.e. the set is an empty set!