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