ARE-INSTANCE-VALUES
===================

syntax: (are-instance-values 
                      <evaluates-to relation-name>
                      <evaluates-to instance-spec>
                      <set>   ;has to be a set!
                      [:read-dependents
                           {T | NIL | Var}]
                      [:if-undetermined
                           {T | NIL | Var}])

This function checks, whether there are particular 
values in the value set of an instance relation. If 
':if-undetermined' is not set to NIL, then a 
'if-undetermined-action' will be started before the 
relation value is found. If ':read-dependents' is 
not set to NIL, then the read-dependents will be 
informed after the relation value is found.

example: >(are-instance-values 'place louvre 
                               {Berlin Paris})
         NIL

         >(are-instance-values 'place louvre {Paris})
         #<set-of T (...)>