a scanner which detects {HEAD ~ a side} of {TAIL ~ a sheet} on which an image is formed using a thermally decolorable coloring agent 
of
a scanner which detects { HEAD ~ a side } of { TAIL ~ a sheet } on which an image is formed using a thermally decolorable coloring agent
OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH