10 CLS
20 SCREEN 1
30 LINE (80,100)-(X,Y),1
31 LINE (80,100)-(X,Y),0
40 X=X-1:Y=Y+1:IF Y=50 THEN GOTO 60
50 GOTO 30
60 LINE (80,100)-(X,Y),1
61 LINE (80,100)-(X,Y),0
62 X=X+1:Y=Y-1:IF X>50 THEN GOTO 30
63 GOTO 60