{HEAD ~ The two flat mirrors} are rotated in {TAIL ~ synchronism} for scanning the light beams longitudinally along the document 
{ HEAD ~ The two flat mirrors } are rotated in { TAIL ~ synchronism } for scanning the light beams longitudinally along the document
HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL REL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH