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