{HEAD ~ a pair} of {TAIL ~ arcuate portions} interconnecting opposed side edges of 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 
of
{ HEAD ~ a pair } of { TAIL ~ arcuate portions } interconnecting opposed side edges of 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 REL 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 OTH OTH OTH OTH OTH OTH OTH OTH