Exercise 4.67: Devise a way to install a loop
detector in the query system so as to avoid the kinds of simple loops
illustrated in the text and in Exercise 4.64.  The general idea is that
the system should maintain some sort of history of its current chain of
deductions and should not begin processing a query that it is already working
on.  Describe what kind of information (patterns and frames) is included in
this history, and how the check should be made.  (After you study the details
of the query-system implementation in 4.4.4, you may want to
modify the system to include your loop detector.)