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(define (julian year month day) (let* ((a (quotient (- 14 month) 12)) (y (+ year 4800 (- a))) (m (+ month (* 12 a) -3))) (+ day (quotient (+ (* 153 m) 2) 5) (* 365 y) (quotient y 4) (- (quotient y 100)) (quotient y 400) (- 32045)))) (define (fri13-ten year month) (let loop ((m (if (= month 12) 1 (+ month 1))) (y (if (= month 12) (+ year 1) year)) (n 0) (k 120)) (if (zero? k) n (loop (if (= m 12) 1 (+ m 1)) (if (= m 12) (+ y 1) y) (if (= (modulo (julian y m 13) 7) 4) (+ n 1) n) (- k 1))))) (display (fri13-ten 2009 3))
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