The block has {HEAD ~ two side extensions} extended inward of {TAIL ~ the notches} of the base 
extended inward of
The block has { HEAD ~ two side extensions } extended inward of { TAIL ~ the notches } of the base
OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH