UNTELL-INSTANCE-VALUES
======================

syntax: (untell-instance-values 
                         <evaluates-to relation-name>
                         <evaluates-to instance-spec>
                         {<set> | _ }
                         [:write-dependents 
                          {T | NIL | Var}]
                         [:probe 
                          {T | NIL | :and-do | Var}])

This function removes all elements of the set from 
the value set of the instance relation. If _ is given 
instead of a set, then the whole value set will be 
replaced with the indefinite empty set. 
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 elements are not included in the value set, 
will be returned as value.

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