Exercise 3.60.  With power series represented as streams of coefficients 
as in exercise 3.59, adding series is implemented by add-streams. 
Complete the definition of the following procedure for multiplying 
series:

(define (mul-series s1 s2)
  (cons-stream <??> (add-streams <??> <??>)))

You can test your procedure by verifying that sin2 x + cos2 x = 1, using 
the series from exercise 3.59.

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(define (mul-series s1 s2)
  (cons-stream (* (stream-car s1) (stream-car s2))
               (add-streams (scale-stream (stream-cdr s2) (stream-car s1)))
                            (mul-series (stream-cdr s1) s2)))