IS-UNDETERMINED-SET
===================

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

        ==> T | NIL

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

Warning: length works on vectors AND lists!

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

         >(is-undetermined-set 
              (ask-instance-values 'landscape Paris))
         T  ; i.e. the set is indefinite and empty!