a-a a-b aaa aab aaf aar aas ab- aba abb abf abr abs af- afa afb aff afr afs ar- ara arb arf arr ars as- asa asb asf asr ass baa bab baf bar bas bba bbb bbf bbr bbs bfa bfb bff bfr bfs bra brb brf brr brs bsa bsb bsf bsr bss faa fab faf far fas fba fbb fbf fbr fbs ffa fff ffr ffs fra frb frf frr frs fsa fsb fsf fsr fss ra- raa rab raf rar ras rba rbb rbf rbr rbs rfa rfb rff rfr rfs rra rrb rrf rrr rrs rsa rsb rsf rsr rss saa sab saf sar sas sba sbb sbf sbr sbs sfa sfb sff sfr sfs sr- sra srb srf srr srs ss- ssa ssb ssf ssr sss