[plt-scheme] frtime slowing down

Hi Greg, all,

Why does the following code slowly use more and more cpu? I'm assuming
that although visually the circles are being removed, something remains
which is getting processed.

(require (lib "animation.ss" "frtime"))

(define (metro tick)
  (let ((tick (floor (* tick 1000))))
    (when-e (> (modulo (floor milliseconds) tick) (/ tick 2)))))

(define (truncate-list lst count)
  (cond 
    ((zero? count) '())
    ((null? lst) '())
    (else (cons (car lst) (truncate-list (cdr lst) (- count 1))))))
 
(display-shapes
 (list
  (collect-b
   (metro 0.1) '()
   (lambda (e lst)
     (cons (make-circle (make-posn 50 (+ 50 (integral 0.2))) 10 "red")
           (truncate-list lst 4))))))

cheers,

dave