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