{TAIL ~ The block} usually includes a plurality of interconnected gas containing tunnels or passages which form {HEAD ~ a closed loop path} in the shape of a triangle, a rectangle, or any polygonal path 
{ TAIL ~ The block } usually includes a plurality of interconnected gas containing tunnels or passages which form { HEAD ~ a closed loop path } in the shape of a triangle , a rectangle , or any polygonal path
TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH