At each service station, a sensor detects {HEAD ~ the presence} of {TAIL ~ a customer} proximate to the station 
At each service station , a sensor detects { HEAD ~ the presence } of { TAIL ~ a customer } proximate to the station
OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH