(letrec ((add (lambda (a b) (+ a b))))
  (write (add 3 4))
  (newline))

;; (letrec ((even? (lambda (n) (if (zero? n) #t (odd? (- n 1)))))
;;          (odd? (lambda (n) (if (zero? n) #f (even? (- n 1))))))
;;   (write (even? 1000))
;;   (newline)
;;   (write (even? 1001))
;;   (newline)
;;   (write (odd? 1000))
;;   (newline)
;;   )

((lambda (even? odd?)
   (set! even? (lambda (n) (if (zero? n) #t (odd? (- n 1)))))
   (set! odd? (lambda (n) (if (zero? n) #f (even? (- n 1)))))
   (write (even? 100))
   (newline)
   (write (even? 101))
   (newline)
   (write (odd? 100))
   (newline)
   )
 'even 'odd)