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