Just as one driving down a highway finds {HEAD ~ his position} by comparing {TAIL ~ viewed landmarks} to a map of the area, the scanned inputs of the return trip provide media density variation landmarks which tell the logic 20 where the media 12 is along its return path 
Just as one driving down a highway finds { HEAD ~ his position } by comparing { TAIL ~ viewed landmarks } to a map of the area , the scanned inputs of the return trip provide media density variation landmarks which tell the logic 20 where the media 12 is along its return path
OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH