This is the fundamental relation for a pair of {HEAD ~ toroidal disks} with {TAIL ~ an intermediate roller} 
with
This is the fundamental relation for a pair of { HEAD ~ toroidal disks } with { TAIL ~ an intermediate roller }
OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL