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