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