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