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