bbb bbc bbo bbr bbs bbt bcb bcc bco bcr bcs bct bob boc boo bor bos bot brb bro brr brs brt bsb bsc bso bsr bss bst btb btc bto btr bts btt c-- cbb cbc cbo cbr cbs cbt ccb ccc cco ccr ccs cct co- cob coc coo cor cos cot crb crc cro crr crs crt csb csc cso csr css cst ctb ctc cto ctr cts ctt ob- obb obc obo obr obs obt oc- ocb occ oco ocr ocs oct oo- oob ooc ooo oor oos oot or- orb orc oro orr ors ort os- osb osc oso osr oss ost ot- otb otc oto otr ots ott rbb rbc rbo rbr rbs rbt rcb rcc rco rcr rcs rct rob roc roo ror ros rot rrb rrc rrr rrs rrt rsb rsc rso rsr rss rst rtb rtc rto rtr rts rtt sbb sbc sbo sbr sbs sbt scb scc sco scr scs sct sob soc soo sor sos sot sr- srb src sro srr srs srt ss- ssb ssc sso ssr sss sst stb stc sto str sts stt tbb tbc tbo tbr tbs tbt tcb tcc tco tcr tcs tct to- tob toc too tor tos tot trb trc tro trr trs trt tsb tsc tso tsr tss tst ttb ttc tto ttr tts ttt