Exercise 3.74: Alyssa P. Hacker is designing a
system to process signals coming from physical sensors.  One important feature
she wishes to produce is a signal that describes the 
zero crossings
of the input signal.  That is, the resulting signal should be 
  
    +
    1
  
 whenever the
input signal changes from negative to positive, 
  
    −
    1
  
 whenever the input signal
changes from positive to negative, and 
  0
 otherwise.  (Assume that the sign of a

  0
 input is positive.)  For example, a typical input signal with its associated
zero-crossing signal would be


… 1 2 1.5 1 0.5 -0.1 -2 -3 -2 -0.5 0.2 3 4 …
… 0 0  0  0  0   -1   0  0  0   0   1  0 0 …


In Alyssa’s system, the signal from the sensor is represented as a stream
sense-data and the stream zero-crossings is the corresponding
stream of zero crossings.  Alyssa first writes a procedure
sign-change-detector that takes two values as arguments and compares the
signs of the values to produce an appropriate 
  0
, 
  1
, or 
  
    −
    1
  
.  She then
constructs her zero-crossing stream as follows:


(define (make-zero-crossings
         input-stream last-value)
  (cons-stream
   (sign-change-detector 
    (stream-car input-stream) 
    last-value)
   (make-zero-crossings 
    (stream-cdr input-stream)
    (stream-car input-stream))))

(define zero-crossings 
  (make-zero-crossings sense-data 0))

Alyssa’s boss, Eva Lu Ator, walks by and suggests that this program is
approximately equivalent to the following one, which uses the generalized
version of stream-map from Exercise 3.50:


(define zero-crossings
  (stream-map sign-change-detector 
              sense-data 
              ⟨expression⟩))

Complete the program by supplying the indicated ⟨expression⟩.