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