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 nth element (counting from 0) 
is n + 1 factorial:

(define factorials (cons-stream 1 (mul-streams <??> <??>)))

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(define (mul-streams a b)
  (stream-map * a b))

(define factorials (cons-stream 1 (mul-streams factorials (stream-cdr integers))))