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