The blocks include openings to define a tunnel extending from top to {HEAD ~ bottom} of {TAIL ~ the stack} 
The blocks include openings to define a tunnel extending from top to { HEAD ~ bottom } of { TAIL ~ the stack }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL