bbb bbc bbo bbr bbs bcb bcc bco bcr bcs bob boc boo bor bos brb bro brr brs bsb bsc bso bsr bss c-- cbb cbc cbo cbr cbs ccb ccc cco ccr ccs co- cob coc coo cor cos crb crc cro crr crs csb csc cso csr css ob- obb obc obo obr obs oc- ocb occ oco ocr ocs oo- oob ooc ooo oor oos or- orb orc oro orr ors os- osb osc oso osr oss rbb rbc rbo rbr rbs rcb rcc rco rcr rcs rob roc roo ror ros rrb rrc rrr rrs rsb rsc rso rsr rss sbb sbc sbo sbr sbs scb scc sco scr scs sob soc soo sor sos sr- srb src sro srr srs ss- ssb ssc sso ssr sss