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

This function removes an element from the value set 
of the instance relation. If ':write-dependents' is 
not set to NIL, then the write-dependents will be 
informed after the value set is changed.
The changed value set or NIL, when the element is not 
included in the value set, will be returned as value.

example: >(untell-instance-value 'offer Paris Louvre)
         #<set-of OFFER (...)>
         >(untell-instance-value 'offer Paris Big-Ben)

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