a-a a-b a-c aaa aab aac aao aar aas ab- aba abb abc abo abr abs ac- aca acb acc aco acr acs aoa aob aoc aoo aor aos ar- ara arb arc aro arr ars as- asa asb asc aso asr ass baa bab bac bao bar bas bba bbb bbc bbo bbr bbs bca bcb bcc bco bcr bcs boa bob boc boo bor bos bra brb bro brr brs bsa bsb bsc bso bsr bss c-- caa cab cac cao car cas cba cbb cbc cbo cbr cbs cca ccb ccc cco ccr ccs co- coa cob coc coo cor cos cra crb crc cro crr crs csa csb csc cso csr css oaa oab oac oao oar oas ob- oba obb obc obo obr obs oc- oca ocb occ oco ocr ocs oo- ooa oob ooc ooo oor oos or- ora orb orc oro orr ors os- osa osb osc oso osr oss ra- raa rab rac rao rar ras rba rbb rbc rbo rbr rbs rca rcb rcc rco rcr rcs roa rob roc roo ror ros rra rrb rrc rrr rrs rsa rsb rsc rso rsr rss saa sab sac sao sar sas sba sbb sbc sbo sbr sbs sca scb scc sco scr scs soa sob soc soo sor sos sr- sra srb src sro srr srs ss- ssa ssb ssc sso ssr sss