RETELL-INSTANCE-VALUE
=====================

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

If an untell-value is included in the value set of 
the instance relation, then it will be replaced 
with the tell-value or with the elements of the 
tell-value, which are not included yet. A value will 
be removed without replacing it, 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 ':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 the 
untell-value is included, otherwise NIL.

example: >(retell-instance-value 'offer Paris 
                                 Moulin-Rouge
                                 Louvre)
         #<set-of OFFER (...)>
         
         >(retell-instance-value 'offer Paris 
                                 {Louvre} 
                                 Centre-Pompidou)
         NIL