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 
  
    
      sin
      2
    
    ⁡
    x
    +
    
      cos
      2
    
    ⁡
    x
    =
    1
    ,
  

using the series from Exercise 3.59.