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