a-a a-b a-t aaa aab aaf aar aat ab- aba abb abf abr abt af- afa afb aff afr aft ar- ara arb arf arr art at- ata atb atf atr att baa bab baf bar bat bba bbb bbf bbr bbt bfa bfb bff bfr bft bra brb brf brr brt bta btb btf btr btt faa fab faf far fat fba fbb fbf fbr fbt ffa fff ffr fft fra frb frf frr frt fta ftb ftf ftr ftt ra- raa rab raf rar rat rba rbb rbf rbr rbt rfa rfb rff rfr rft rra rrb rrf rrr rrt rta rtb rtf rtr rtt taa tab taf tar tat tba tbb tbf tbr tbt tfa tfb tff tfr tft tra trb trf trr trt tta ttb ttf ttr ttt