(define while
  ; TYPE: (-> boolean), (-> void) -> void
  (lambda (test body)
    ; EFFECT: do the test, if it's true
    ; do the body; repeat until
    ; the test is false.
    ; NOTE: presumably the body changes something
    ; that the test checks
    (letrec ((loop ; TYPE: -> void
	       (lambda ()
		 (if (test)
		     (begin
		       (body)
		       (loop))))))
      (loop))))

(define until
  ; TYPE: (-> boolean), (-> void) -> void
  (lambda (test body)
    (while (lambda () (not (test)))
	   body)))