{HEAD ~ The propped block} 31 is formed with {TAIL ~ spaced horizontal propped surfaces} 312, and 313 at different heights 
is formed with
{ HEAD ~ The propped block } 31 is formed with { TAIL ~ spaced horizontal propped surfaces } 312 , and 313 at different heights
HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH