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