10 CLS
20 SCREEN 1
29 NOISE 4,15,1
30 LINE (80,100)-(X,Y),1
31 IF Y<289 THEN LINE (80,100)-(X,Y),0
40 X=X-9:Y=Y+9:IF Y>290 THEN GOTO 59
50 GOTO 30
59 NOISE 4,15,1
60 LINE (80,100)-(X,Y),2
61 LINE (80,100)-(X,Y),0
62 X=X+9:Y=Y-9:IF X>290 THEN GOTO 29
63 GOTO 60