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