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