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syntax: (retell-instance-values 
                        <evaluates-to relation-name>
                        <evaluates-to instance-spec>
                        {<lisp-type: T> | <set>}
                           ;; Tell-value
                        {<set> | _ }
                           ;; Untell-value
                           {T | NIL | Var}]
                           {T | NIL | :and-do | Var}])

If elements of the untell-value are included in the 
value set of the instance relation, then the value 
will be replaced with the tell-value or with 
the elements of the tell-value, which are not 
included yet. Values will be removed without 
replacing them, when the tell-value is part of the 
value set. If the untell-value is not included in 
the value set, then it can not be replaced.
If '_' is given instead of the untell-value, then the 
whole value set will be replaced with the tell-
value. If ':write-dependents' is not set to NIL, then 
the write-dependents will be informed after the 
untell-value was replaced.
The changed value set will be returned, when elements 
of the untell-value are included, otherwise NIL.

example: >(retell-instance-values 'offer Paris 
         #<set-of OFFER (...)>

         >(retell-instance-values 'offer Paris
                                  {Moulin Rouge} 

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