TELL-INSTANCE-VALUE
===================

syntax: (tell-instance-value 
                        <evaluates-to relation-name>
                        <evaluates-to instance-spec>
                        <lisp-type: T>
                        [:write-dependents 
                           {T | NIL | Var}]
                        [:probe 
                           {T | NIL | :and-do | Var}])

This function adds an element to the value set of 
the instance relation, when it is not included yet. 
If ':write-dependents' is not set to NIL, then the 
write-dependents will be informed after the value set 
is changed. The value set will be returned, when it 
has been changed, otherwise NIL will be returned.

example: >(tell-instance-value 'offer Paris Louvre)
         NIL

         >(tell-instance-value 'offer Paris 
                               Centre-Pompidou)
         #<set-of OFFER (...)>