Just as one driving down a highway finds his position by comparing 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 {HEAD ~ the media} 12 is along {TAIL ~ its return path} 
is along
Just as one driving down a highway finds his position by comparing 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 { HEAD ~ the media } 12 is along { TAIL ~ its return path }
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 OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL