Exercise 3.76.  Eva Lu Ator has a criticism of Louis's approach in 
exercise 3.75. The program he wrote is not modular, because it 
intermixes the operation of smoothing with the zero-crossing extraction. 
For example, the extractor should not have to be changed if Alyssa finds 
a better way to condition her input signal. Help Louis by writing a 
procedure smooth that takes a stream as input and produces a stream in 
which each element is the average of two successive input stream 
elements. Then use smooth  as a component to implement the zero-crossing 
detector in a more modular style.

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(define (smooth signal)
  (stream-map avg signal
                  (stream-cdr signal)))

or:

(define (smooth signal)
  (stream-map avg signal
                  (cons-stream (stream-car signal)
                               signal)))

(define (make-zero-crossings input-stream)
  (let ((smoothed (smooth input-stream)))
    (stream-map sign-change-detector smoothed (stream-cdr smoothed))))