The first on disc position and the second on disc position are compared to decide whether {HEAD ~ an unused portion} is present in or absent from {TAIL ~ a region therebetween} 
is absent from
The first on disc position and the second on disc position are compared to decide whether { HEAD ~ an unused portion } is present in or absent from { TAIL ~ a region therebetween }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL OTH OTH OTH REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL