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