10 CLS : KEY OFF
20 SCREEN 9
30 C=1
40 FOR A=1 TO 345
50 LINE (645,A)-(0,350-A),C
60 C=C+1: IF C=15 THEN C=0
70 NEXT A
80 FOR A= 1 TO 645
90 LINE (A,350)-(645-A,0),C
100 C=C+1: IF C=15 THEN C=0
110 NEXT A