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diff --git a/semestr-2/racket/l9z16/solution.rkt b/semestr-2/racket/l9z16/solution.rkt
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+#lang racket

+

+(provide lcons lnull lnull? lcar lcdr)

+

+

+(define (lcons x f) (mcons x f))

+

+(define lnull null)

+

+(define lnull? null?)

+

+(define (lcar xs) (mcar xs))

+

+(define (lcdr xs) 

+  (let ([x (mcdr xs)])

+    (cond [(not (mpair? x)) (set-mcdr! xs (x))]))

+  (mcdr xs))

+

+(define (from n)

+  (lcons n (lambda () (from (+ n 1)))))

+

+(define nats

+  (from 0))

+

+(define (lnth n xs)

+  (cond [(= n 0) (lcar xs)]

+        [else (lnth (- n 1) (lcdr xs))]))

+

+(define (lfilter p xs)

+  (cond [(lnull? xs) lnull]

+        [(p (lcar xs))

+         (lcons (lcar xs) (lambda () (lfilter p (lcdr xs))))]

+        [else (lfilter p (lcdr xs))]))

+

+(define (prime? n)

+  (define (factors i)

+    (cond [(>= i n) (list n)]

+          [(= (modulo n i) 0) (cons i (factors (+ i 1)))]

+          [else (factors (+ i 1))]))

+  (= (length (factors 1)) 2))

+

+(define primes (lfilter prime? (from 2)))
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