GET:PHYSICA_keywords:FILL

examples

 FILL=34 means to light up every 3rd dot horizontally and every
 4th dot vertically.  FILL=-34 means to erase every 3rd dot
 horizontally and every 4th dot vertically.  FILL=8 means to fill
 with hatch pattern 8. FILL=-8 means to erase using hatch pattern 8.