The unit may also have {HEAD ~ a tubing} to allow {TAIL ~ oxygen} or another treated gas to come in 
The unit may also have { HEAD ~ a tubing } to allow { TAIL ~ oxygen } or another treated gas to come in
OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL REL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH