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