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