The extent of {TAIL ~ opening} of {HEAD ~ the sectors} 12a and 12b and the count N of the up down counter 42 have a pre established relationship 
The extent of { TAIL ~ opening } of { HEAD ~ the sectors } 12a and 12b and the count N of the up down counter 42 have a pre established relationship
OTH OTH OTH TAIL TAIL TAIL TAIL TAIL OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH