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