>>> (define (f x) (+ x 6))
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>>> (define str (cons-stream (f 1) (cons-stream (f 2) str)))
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>>> (car str)
______
>>> (cdr str)
______
>>> (cdr-stream str)
______
>>> (cdr str)
______
>>> (define (f x) (+ x 8))
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>>> (cdr-stream str)
______
>>> (cdr-stream (cdr-stream str))
______
>>> (define (f x) (- x 1))
______
>>> (cdr-stream (cdr-stream (cdr-stream str)))
______