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syntax: (ask-instance-value 
                        <evaluates-to relation-name>
                        <evaluates-to instance-spec>
                             {T | NIL | Var}]
                             {T | NIL | Var}])

This function returns the first element of the value
set of an instance relation or NIL. If ':if-undeter-
mined' 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 rela-
tion value is found.

example: >(ask-instance-value 'place louvre)
         #<capital-city Paris>

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