IS-INSTANCE-VALUE
=================

syntax: (is-instance-value 
                        <evaluates-to relation-name>
                        <evaluates-to instance-spec>
                        <lisp-type: T>
                        [:read-dependents
                            {T | NIL | Var}]
                        [:if-undetermined
                            {T | NIL | Var}])

This function checks, whether there is a particular 
value in the value set of the 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: >(is-instance-value 'place louvre Berlin)
         NIL

         >(is-instance-value 'place louvre Paris)
         #<capital-city Paris>