a-a a-b a-t aaa aab aar aas aat ab- aba abb abr abs abt ar- ara arb arr ars art as- asa asb asr ass ast at- ata atb atr ats att baa bab bar bas bat bba bbb bbr bbs bbt bra brb brr brs brt bsa bsb bsr bss bst bta btb btr bts btt ra- raa rab rar ras rat rba rbb rbr rbs rbt rra rrb rrr rrs rrt rsa rsb rsr rss rst rta rtb rtr rts rtt saa sab sar sas sat sba sbb sbr sbs sbt sr- sra srb srr srs srt ss- ssa ssb ssr sss sst sta stb str sts stt taa tab tar tas tat tba tbb tbr tbs tbt tra trb trr trs trt tsa tsb tsr tss tst tta ttb ttr tts ttt