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