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