a-a a-b a-t aaa aab aaf aao aat ab- aba abb abf abo abt af- afa afb aff afo aft aoa aob aof aoo aot at- ata atb atf ato att baa bab baf bao bat bba bbb bbf bbo bbt bfa bfb bff bfo bft boa bob bof boo bot bta btb btf bto btt faa fab faf fao fat fba fbb fbf fbo fbt ffa fff ffo fft foa fob fof foo fot fta ftb ftf fto ftt oaa oab oaf oao oat ob- oba obb obf obo obt of- ofa ofb off ofo oft oo- ooa oob oof ooo oot ot- ota otb otf oto ott taa tab taf tao tat tba tbb tbf tbo tbt tfa tfb tff tfo tft to- toa tob tof too tot tta ttb ttf tto ttt