{HEAD ~ a single block} composed of {TAIL ~ catalyzed acrylic resin} and having a pair of prism boxes inversely positioned and interconnected by a pair of posterior walls provided with a window, 
composed of
{ HEAD ~ a single block } composed of { TAIL ~ catalyzed acrylic resin } and having a pair of prism boxes inversely positioned and interconnected by a pair of posterior walls provided with a window ,
HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH