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