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