Exercise 3.54: Define a procedure
mul-streams, analogous to add-streams, that produces the
elementwise product of its two input streams.  Use this together with the
stream of integers to complete the following definition of the stream
whose 
  
    n
    
      th
    
  
 element (counting from 0) is 
  
    n
    +
    1
  
 factorial:


(define factorials 
  (cons-stream 1 (mul-streams ⟨??⟩ ⟨??⟩)))