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