SICP Exercise 3.33 constraint propagation averager

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.

 SOLUTION

 The code is here.

 The picture below shows how the constraints and connectors are assembled:

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