(define my-prime-stream
  (let ((prime?
	 (lambda(n)
	   (define-stream (iter s)
	     (let-stream ((np (stream-car s)))
			 (cond
			  ((> np (sqrt n)) (quote true))
			  ((= 0 (modulo n np)) #f)
			  (else (iter (stream-cdr s))))))
	   (iter my-prime-stream))))
  (stream-cons 2 (stream-filter prime? natural-numbers-stream))))