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