SICP in Clojure: Chapter 1, Exercise 24

This is my Clojure solution to Chapter 1, Exercise 24:

(ns sicp.ch1.ex24
  (:require [sicp.ch1.ex16 :as sicp-ch1-ex16]
            [sicp.ch1.ex22 :as sicp-ch1-ex22]))

(defn expmod
  [base exp m]
  (cond (zero? exp) 1
        (even? exp) (rem (sicp-ch1-ex16/square (expmod base
                                                       (/ exp 2)
                                                       m))
                         m)
        :else       (rem (* base (expmod base (dec exp) m))
                         m)))

(defn fermat-test
  [n]
  (letfn [(try-it [a]
            (= (expmod a n n) a))]
    (try-it (inc (rand-int (dec n))))))

(defn fast-prime?
  [n times]
  (cond (zero? times)   true
        (fermat-test n) (fast-prime? n (dec times))
        :else           false))

(defn start-prime-test
  [n start-time]
  (let [p (fast-prime? n 10)]
    (when p
      (sicp-ch1-ex22/report-prime n (- (sicp-ch1-ex22/runtime)
                                       start-time)))
    p))

(defn timed-prime-test
  [n]
  (start-prime-test n (sicp-ch1-ex22/runtime)))

(def primes [1009 1013 1019 10007 10009 10037 100003 100019 100043
             1000003 1000033 1000037])

(map sicp-ch1-ex22/timed-prime-test primes)

(map timed-prime-test primes)

(def old-times [131134 49417 48302 153392 159432 167507 523204 498113
                357480 815095 580023 253359])

(def new-times [461622 142283 171243 156552 274436 89991 78386 77681
                74216 99555 79155 85534])

(map #(double (/ %1 %2)) old-times new-times) ; 0.28--8.19x (dis)improvement