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