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