The gas may be used as a single type thereof or as {HEAD ~ combinations} of {TAIL ~ different gases} 
of
The gas may be used as a single type thereof or as { HEAD ~ combinations } of { TAIL ~ different gases }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL