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