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