10 line 170,90,470,90
20 line 470,90,470,390
30 line 470,390,170,390
40 line 170,390,170,90
1000  for l=1 to 29
1010   line 170+10*l,90,470,90+10*l
1020  next l
1030  for l=1 to 29
1040   line 470,90+10*l,470-10*l,390
1050  next l
1060  for l=1 to 29
1070   line 470-10*l,390,170,390-10*l
1080  next l
1090  for l=1 to 29
1100   line 170,390-10*l,170+10*l,90
1110  next l
5000 cls 1
5010 list