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