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