{HEAD ~ means} for sensing {TAIL ~ a start} of a line image to generate said clear signal; and 
{ HEAD ~ means } for sensing { TAIL ~ a start } of a line image to generate said clear signal ; and
HEAD HEAD HEAD HEAD HEAD REL REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH