{HEAD ~ The block} is usually filled with {TAIL ~ a lasing gas} such as a mixture of helium and neon 
is usually filled with
{ HEAD ~ The block } is usually filled with { TAIL ~ a lasing gas } such as a mixture of helium and neon
HEAD HEAD HEAD HEAD HEAD HEAD REL REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH