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 section 4.4.4, you may 
want to modify the system to include your loop detector.)

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When processing a query, the system can record the pattern and the 
current frame on a stack of active (sub-)queries.  If the current 
pattern is already on this stack, it should signal an error if the frame 
contains the same or fewer bindings than the frame already on the stack.

As long as the sub-query is more constrained, i.e. has more variables 
bound than the existing query, then the query is still making progress 
and can proceed.