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>.

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(define zero-crossings
  (stream-map sign-change-detector sense-data (stream-cdr sense-data)))