bbb bbc bbk bbo bbs bcb bcc bck bco bcs bkb bkc bkk bko bks bob boc bok boo bos bsb bsc bsk bso bss c-- cbb cbc cbk cbo cbs ccb ccc cck cco ccs ckb ckc cko cks co- cob coc cok coo cos csb csc csk cso css kbb kbc kbk kbo kbs kcb kcc kck kcs kkb kkc kkk kks kob koc kok koo kos ksb ksc ksk kso kss ob- obb obc obk obo obs oc- ocb occ ock oco ocs okb okc okk oko oks oo- oob ooc ook ooo oos os- osb osc osk oso oss sbb sbc sbk sbo sbs scb scc sck sco scs sk- skb skc skk sko sks sob soc sok soo sos ss- ssb ssc ssk sso sss