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