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