Hello everyone, #|  i will be as brief as i can in explainig the problem. this is the original code of exercise-6.6.6 copied as it is from online version & also from the printed book: (draw-a-circle   (move-circle 10     (move-circle 10       (move-circle 10         (move-circle 10 ... a circle ...))))) ;; the last line says ... a circle....  without giving any hint what "a circle" is . i think it is a printing mistake and it should be "...a-circle...". even after changing it to "a-circle"  the function does not work.  (draw-a-circle (move-circle 10 (move-circle 10 (move-circle 10 (move-circle 10 a-circle))))) and i got following ouput: ------------------------------------------- Welcome to DrScheme, version 209. Language: Beginning Student. Teachpack: /usr/lib/plt/teachpack/htdp/draw.ss. true color-circle-center: expects argument of type <struct:color-circle>; given true > -------------------------------------------------- "true" is from the innermost expression  as it moves the cicle 1st time but next lies error. it does not move the circle 4 times as told in the book. I think i know the reason. from my common sense  (move-circle 10 a-circle) produces not the circle but "true" & hence "true" goes intop drscheme not the circle itself . I have  verified this thing from "stepper" too. since (move-circle..) did not get its 2nd parameter as structue it reports an error for it got "true" - the boolean. i am using DrScheme version 209 on Debian "sarge".i got 209 version with the debian's 14 CD packs, but i also tried "plt-209-bin-i386-linux.sh" onto Fedora Core but again the same problem. I have studied only upto chapter 6 of HTDP and i do not know any other language, so please keep the answer according to the HTDP's way.  may you help me? thanks in advance |# ------------------- here is the programme ----------------------------------- (define-struct color-circle (center radius color)) ;; A color-cirlce is a structure: (make-color-circle cen r col) ;; where cen is a posn structure, r is a number and col is a symbol. ;; in-circle? : a-color-circle, a-posn -> boolean ;; to deiermine whether the point lies inside the circle or not. #| Template: (define (fun-for-circle a-circle a-posn)   ...(color-circle-center a-circle)...   ...(color-circle-radius a-circle)...   ...(color-circle-color a-circle)...) |# (define (move-circle delta a-circle)   (cond     [(draw-and-clear-circle a-circle) (translate-circle a-circle delta)]     [else a-circle])) (define (translate-circle a-color-circle delta)   (draw-circle (make-posn (+ delta (posn-x (color-circle-center a-color-circle)))                           (posn-y (color-circle-center a-color-circle)))                (color-circle-radius a-color-circle)                (color-circle-color a-color-circle))) (define (draw-and-clear-circle a-circle)   (and (draw-a-circle a-circle)        (sleep-for-a-while 1)        (clear-a-circle a-circle))) (define (draw-a-circle a-circle)   (draw-circle (color-circle-center a-circle)                (color-circle-radius a-circle)                (color-circle-color a-circle))) (define (clear-a-circle a-circle)   (clear-circle (color-circle-center a-circle)                 (color-circle-radius a-circle)                 (color-circle-color a-circle))) ;; test (define a-circle (make-color-circle (make-posn 30 30) 10 'red)) (start 300 300) (draw-a-circle (move-circle 10 (move-circle 10 (move-circle 10 a-circle)))) --------------------- end of programme -------------------------------------