sensing, at each service station, a change of state resulting from {HEAD ~ a customer} being proximate, or not, to {TAIL ~ the station} 
proximate to
sensing , at each service station , a change of state resulting from { HEAD ~ a customer } being proximate , or not , to { TAIL ~ the station }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH REL OTH OTH OTH OTH REL TAIL TAIL TAIL TAIL TAIL TAIL