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