Guessing game from Realm of Racket

guessing.rkt

#lang racket

(require rackunit rackunit/text-ui)

(define (guess upper lower)
  (quotient (+ upper lower) 2))

(define (smaller upper lower)
  (guess
    (max lower
         (sub1 (guess upper lower)))
    lower))

(define (bigger upper lower)
  (guess
    upper
    (min upper
         (add1 (guess upper lower)))))

(run-tests
  (test-suite
   "guessing game"
   (check-equal? (guess 100 1) 50)
   (check-equal? (smaller 100 1) 25)
   (check-equal? (bigger 100 1) 75)))