a-a a-b a-t aaa aab aao aas aat ab- aba abb abo abs abt aoa aob aoo aos aot as- asa asb aso ass ast at- ata atb ato ats att baa bab bao bas bat bba bbb bbo bbs bbt boa bob boo bos bot bsa bsb bso bss bst bta btb bto bts btt oaa oab oao oas oat ob- oba obb obo obs obt oo- ooa oob ooo oos oot os- osa osb oso oss ost ot- ota otb oto ots ott saa sab sao sas sat sba sbb sbo sbs sbt soa sob soo sos sot ss- ssa ssb sso sss sst sta stb sto sts stt taa tab tao tas tat tba tbb tbo tbs tbt to- toa tob too tos tot tsa tsb tso tss tst tta ttb tto tts ttt