Exercise 4.2
A stream of factorial numbers **

Make a stream of all factorial numbers: 1 2 6 24 120 ...

Please go for a recursive definition, in the same style as used for nat-nums and fib:

(define ones (cons-stream 1 ones))

(define nat-nums 
 (cons-stream 1 
  (add-streams ones nat-nums)))

(define fibs
  (cons-stream 0
    (cons-stream 1
      (add-streams (tail fibs) fibs))))

As part of the solution, you may need a multiplicative stream function similar to add-streams. For that purpose make a higher-order function, combine-streams, that combines two (infinite) streams with a binary function.

Here are all the necessary definitions you need to get started:

(define-syntax cons-stream
  (syntax-rules ()
    ((cons-stream x y)
     (cons x (delay y)))))

(define head car)

(define (tail stream) (force (cdr stream)))

(define empty-stream? null?)

(define the-empty-stream '())


(define (stream-section n stream)
  (cond ((= n 0) '())
        (else (cons (head stream)
              (stream-section 
                (- n 1)
               (tail stream))))))

(define (add-streams s1 s2)
 (let ((h1 (head s1))
       (h2 (head s2)))
   (cons-stream 
    (+ h1 h2)
    (add-streams (tail s1) (tail s2)))))


(define ones (cons-stream 1 ones))

(define nat-nums 
 (cons-stream 1 
  (add-streams ones nat-nums)))