(define (-& x y k) (k (- x y)))
(define (zero?& x k) (k (zero? x)))
(define (halt x) x)

(letrec ((even? (lambda (n ktail)
                  (zero?& n
                          (lambda (x1)
                            (if x1
                                #t
                                (-& n 1
                                    (lambda (x2)
                                      (odd? x2 ktail))))))))
         (odd? (lambda (n ktail)
                 (zero?& n
                         (lambda (x1)
                           (if x1
                               #f
                               (-& n 1
                                   (lambda (x2)
                                     (even? x2 ktail)))))))))
  (even? 12 halt))