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.