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LL Golf Hole 3 - 13日の金曜日を数え上げる (Nested Flatten)
SERIESとMETATILITIESを利用してみました。

;>>> Friday, February 13, 2009
;>>> Friday, March 13, 2009
;>>> Friday, November 13, 2009
;>>> Friday, August 13, 2010
;>>> Friday, May 13, 2011
;>>> Friday, January 13, 2012
;>>> Friday, April 13, 2012
;>>> Friday, July 13, 2012
;>>> Friday, September 13, 2013
;>>> Friday, December 13, 2013
;==> 10
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(use-package :series)

(defun friday13-p (ut)
  (and (= 13 (nth-value 3 (decode-universal-time ut)))
       (= 4 (nth-value 6 (decode-universal-time ut)))))

(let* ((from (get-universal-time))
       (to (encode-universal-time 59 59 23 31 12 2013))
       (uts (choose-if #'friday13-p (scan-range :from from :upto to :by (* 60 60 24)))))
  (iterate ((ut uts)) 
    (format t "~A~%" (metatilities:date-string ut)))
  (collect-length uts))

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