{TAIL ~ At least three sensors} are arranged in {HEAD ~ a substantially regular manner} essentially along a closed curve 
{ TAIL ~ At least three sensors } are arranged in { HEAD ~ a substantially regular manner } essentially along a closed curve
TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH