In section we saw that not and lisp-value can cause the query language togive ‘‘wrong’’ answers if these filtering operations are applied to frames in which variables areunbound. Devise a way to fix this shortcoming. One idea is to perform the filtering in a ‘‘delayed’’manner by appending to the frame a ‘‘promise’’ to filter that is fulfilled only when enough variableshave been bound to make the operation possible. We could wait to perform filtering until all otheroperations have been performed. However, for efficiency’s sake, we would like to perform filtering assoon as possible so as to cut down on the number of intermediate frames generated.