A detector at {HEAD ~ the entrance} of {TAIL ~ a defined area} detects passage of a customer at the entrance 
of
A detector at { HEAD ~ the entrance } of { TAIL ~ a defined area } detects passage of a customer at the entrance
OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH