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