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