bbb bbo bbr bbs bbt bob boo bor bos bot brb bro brr brs brt bsb bso bsr bss bst btb bto btr bts btt ob- obb obo obr obs obt oo- oob ooo oor oos oot or- orb oro orr ors ort os- osb oso osr oss ost ot- otb oto otr ots ott rbb rbo rbr rbs rbt rob roo ror ros rot rrb rrr rrs rrt rsb rso rsr rss rst rtb rto rtr rts rtt sbb sbo sbr sbs sbt sob soo sor sos sot sr- srb sro srr srs srt ss- ssb sso ssr sss sst stb sto str sts stt tbb tbo tbr tbs tbt to- tob too tor tos tot trb tro trr trs trt tsb tso tsr tss tst ttb tto ttr tts ttt