a-a a-b a-t aaa aab aaf aan aat ab- aba abb abf abn abt af- afa afb aff afn aft an- ana anb anf ann ant at- ata atb atf atn att baa bab baf ban bat bba bbb bbf bbn bbt bfa bfb bff bfn bft bna bnb bnf bnn bnt bta btb btf btn btt faa fab faf fan fat fba fbb fbf fbn fbt ffa fff ffn fft fna fnb fnf fnn fnt fta ftb ftf ftn ftt naa nab naf nan nat nba nbb nbf nbn nbt nfa nfb nff nfn nft nna nnb nnf nnn nnt nta ntb ntf ntn ntt taa tab taf tan tat tba tbb tbf tbn tbt tfa tfb tff tfn tft tna tnb tnf tnn tnt tta ttb ttf ttn ttt