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