; compare Program 11.8, pg. 353 -
(define while-proc
  ; TYPE: (-> ((-> () boolean) (-> () void))
  ;           void)
  (lambda (pred-th body-th)
    ; EFFECT: do the pred-th, if it's true do the body-th;
    ; repeat until the pred-th is false.
    (letrec ((loop ; TYPE: (-> () void)
	       (lambda ()
		 (if (pred-th)
		     (begin
		       (body-th)
		       (loop))))))
      (loop))))