#lang racket
(define (mod-op op a b n)
  (modulo (op 
           (modulo a n)
           (modulo b n))
          n))
(define (mod-exp a x n)
  (cond ((zero? x) 1)
        ((odd? x)
         (mod-op * (mod-exp (mod-op * a a n) (quotient x 2) n) a n))
        ((even? x)
         (mod-exp (mod-op * a a n) (quotient x 2) n))))