{HEAD ~ Recess} 42 in turn comprises {TAIL ~ channels} 44 and open portion 46 
comprises
{ HEAD ~ Recess } 42 in turn comprises { TAIL ~ channels } 44 and open portion 46
HEAD HEAD HEAD HEAD HEAD OTH OTH OTH REL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH