Exercise 4.66.  Ben has been generalizing the query system to provide 
statistics about the company. For example, to find the total salaries of 
all the computer programmers one will be able to say

(sum ?amount
     (and (job ?x (computer programmer))
          (salary ?x ?amount)))

In general, Ben's new system allows expressions of the form

(accumulation-function <variable>
                       <query pattern>)

where accumulation-function can be things like sum, average, or maximum. 
Ben reasons that it should be a cinch to implement this. He will simply 
feed the query pattern to qeval. This will produce a stream of frames. 
He will then pass this stream through a mapping function that extracts 
the value of the designated variable from each frame in the stream and 
feed the resulting stream of values to the accumulation function. Just 
as Ben completes the implementation and is about to try it out, Cy walks 
by, still puzzling over the wheel query result in exercise 4.65. When Cy 
shows Ben the system's response, Ben groans, ``Oh, no, my simple 
accumulation scheme won't work!''

What has Ben just realized? Outline a method he can use to salvage the 
situation. 

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As in Cy's query, results may contain duplicates which vary in some 
assignments but not others.  This will make Ben's implementation of sum 
or average give incorrect results.

A solution is to specify a way of restricting the results of a query to 
values which are unique in a particular variable.  With such a 
primitive, Ben could rewrite his query as:

(sum ?amount
     (unique ?x
       (and (job ?x (computer programmer))
            (salary ?x ?amount))))

And Cy's query could be written as

(unique ?who (wheel ?who))