{TAIL ~ The air vent} is provided in a vent pipe of the system above a trap of {HEAD ~ the system} 
{ TAIL ~ The air vent } is provided in a vent pipe of the system above a trap of { HEAD ~ the system }
TAIL 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