Exercise 3.33.  Using primitive multiplier, adder, and constant 
constraints, define a procedure averager that takes three connectors a, 
b, and c as inputs and establishes the constraint that the value of c is 
the average of the values of a and b.

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(define (averager a b c)
  (let ((u (make-connector))
        (v (make-connector)))
    (adder a b u)
    (multiplier c v u)
    (constant 2 v)
    'ok))