bbb bbk bbo bbr bbs bkb bkk bko bkr bks bob bok boo bor bos brb brk bro brr brs bsb bsk bso bsr bss kbb kbk kbo kbr kbs kkb kkk kkr kks kob kok koo kor kos krk kro krr krs ksb ksk kso ksr kss ob- obb obk obo obr obs okb okk oko okr oks oo- oob ook ooo oor oos or- orb ork oro orr ors os- osb osk oso osr oss rbb rbk rbo rbr rbs rkb rkk rko rks rob rok roo ror ros rrb rrk rrr rrs rsb rsk rso rsr rss sbb sbk sbo sbr sbs sk- skb skk sko skr sks sob sok soo sor sos sr- srb srk sro srr srs ss- ssb ssk sso ssr sss