{HEAD ~ a pair} of arcuate portions interconnecting opposed side edges of {TAIL ~ said flat portions}, respectively, one of said arcuate portions being formed with a pair of engage grooves on an outer surface thereof so as to extend longitudinally of said arcuate portion, and 
{ HEAD ~ a pair } of arcuate portions interconnecting opposed side edges of { TAIL ~ said flat portions } , respectively , one of said arcuate portions being formed with a pair of engage grooves on an outer surface thereof so as to extend longitudinally of said arcuate portion , and
HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH