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