<div style="font-family: 'Times New Roman'; font-size: 16px;">Hello, I have this programm : ; WorldState -> WorldState ; the clock ticked; decrease the gauge with 0.1 ; example: ; given: 20, expected 19.9 ; given: 78, expected 77.9 (define (tock ws) (if (> ws 0) (- ws 0.1)0 )) ; GaugeState -> Gaugestate. ; display the gauge. (define (gauge ws) (overlay/xy (rectangle 100 20 "outline" "black") 0 0 (rectangle ws 20 "solid" "red"))) ; World -> World ; adjust the gauge when the user uses the up or down key. ; given 20 and the down-key expected 20.5 ; given 78 and the up-key expected 78,3 (define (toets ws input) (cond [ (key=? input "down") (+ ws 0.3)] [ (key=? input "up") (+ ws 0.5)] [ else ws] )) (define (main ws) (big-bang ws (on-tick tock) (to-draw gauge) (on-key toets))) It runs perfectly but do I understand it right that the variable ws is the same as world. Roelof </div>