Exercise 3.75: Unfortunately, Alyssa’s
zero-crossing detector in Exercise 3.74 proves to be insufficient,
because the noisy signal from the sensor leads to spurious zero crossings.  Lem
E.  Tweakit, a hardware specialist, suggests that Alyssa smooth the signal to
filter out the noise before extracting the zero crossings.  Alyssa takes his
advice and decides to extract the zero crossings from the signal constructed by
averaging each value of the sense data with the previous value.  She explains
the problem to her assistant, Louis Reasoner, who attempts to implement the
idea, altering Alyssa’s program as follows:


(define (make-zero-crossings 
         input-stream last-value)
  (let ((avpt 
         (/ (+ (stream-car input-stream) 
               last-value) 
            2)))
    (cons-stream 
     (sign-change-detector avpt last-value)
     (make-zero-crossings 
      (stream-cdr input-stream) avpt))))

This does not correctly implement Alyssa’s plan.  Find the bug that Louis has
installed and fix it without changing the structure of the program.  (Hint: You
will need to increase the number of arguments to make-zero-crossings.)