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