a-a a-b a-c aaa aab aac aaf aak aao aar aas ab- aba abb abc abf abk abo abr abs ac- aca acb acc acf ack aco acr acs af- afa afb afc aff afk afo afr afs aka akb akc akk ako akr aks aoa aob aoc aof aok aoo aor aos ar- ara arb arc arf ark aro arr ars as- asa asb asc asf ask aso asr ass baa bab bac baf bak bao bar bas bba bbb bbc bbf bbk bbo bbr bbs bca bcb bcc bcf bck bco bcr bcs bfa bfb bfc bff bfo bfr bfs bka bkb bkc bkf bkk bko bkr bks boa bob boc bof bok boo bor bos bra brb brf brk bro brr brs bsa bsb bsc bsf bsk bso bsr bss c-- caa cab cac caf cak cao car cas cba cbb cbc cbf cbk cbo cbr cbs cca ccb ccc ccf cck cco ccr ccs cfa cfb cfc cff cfk cfo cfr cfs cka ckb ckc cko cks co- coa cob coc cof cok coo cor cos cra crb crc crf crk cro crr crs csa csb csc csf csk cso csr css faa fab fac faf fak fao far fas fba fbb fbc fbf fbk fbo fbr fbs fca fcb fcc fcf fck fco fcr fcs ffa ffc fff ffk ffo ffr ffs fka fkb fkc fkf fkk foa fob foc fof fok foo for fos fra frb frc frf frk fro frr frs fsa fsb fsc fsf fsk fso fsr fss ka- kaa kab kac kaf kak kao kar kas kba kbb kbc kbf kbk kbo kbr kbs kca kcb kcc kck kcr kcs kfa kfc kff kfo kfs kkb kkc kkk kkr kks koa kob koc kof kok koo kor kos kra krc krf krk kro krr krs ksa ksb ksc ksf ksk kso ksr kss oaa oab oac oaf oak oao oar oas ob- oba obb obc obf obk obo obr obs oc- oca ocb occ ocf ock oco ocr ocs of- ofa ofb ofc off ofo ofr ofs oka okb okc okk oko okr oks oo- ooa oob ooc oof ook ooo oor oos or- ora orb orc orf ork oro orr ors os- osa osb osc osf osk oso osr oss ra- raa rab rac raf rak rao rar ras rba rbb rbc rbf rbk rbo rbr rbs rca rcb rcc rcf rco rcr rcs rfa rfb rfc rff rfk rfo rfr rfs rkb rkc rkf rkk rko rks roa rob roc rof rok roo ror ros rra rrb rrc rrf rrk rrr rrs rsa rsb rsc rsf rsk rso rsr rss saa sab sac saf sak sao sar sas sba sbb sbc sbf sbk sbo sbr sbs sca scb scc scf sck sco scr scs sfa sfb sfc sff sfk sfo sfr sfs sk- ska skb skc skf skk sko skr sks soa sob soc sof sok soo sor sos sr- sra srb src srf srk sro srr srs ss- ssa ssb ssc ssf ssk sso ssr sss