<a href="http://projecteuler.net/index.php?section=problems&id=188">Problem 188</a>

タワー表記かよ. 実はあっさり解ける問題.

(define (find a b n)
  (define (order a n)
    (let loop ((i 1) (c a))
      (if (= c 1) i
        (loop (+ i 1) (remainder (* c a) n)))))
  (if (= b 1) (remainder a n)
    (let1 o (order a n)
          (expt-mod a (find a (- b 1) o) n))))
;(time (find 1777 1855 (expt 10 8)))
; real   1.938
; user   1.719
; sys    0.219