a-a a-b a-t aaa aab aaf aas aat ab- aba abb abf abs abt af- afa afb aff afs aft as- asa asb asf ass ast at- ata atb atf ats att baa bab baf bas bat bba bbb bbf bbs bbt bfa bfb bff bfs bft bsa bsb bsf bss bst bta btb btf bts btt faa fab faf fas fat fba fbb fbf fbs fbt ffa fff ffs fft fsa fsb fsf fss fst fta ftb ftf fts ftt saa sab saf sas sat sba sbb sbf sbs sbt sfa sfb sff sfs sft ss- ssa ssb ssf sss sst sta stb stf sts stt taa tab taf tas tat tba tbb tbf tbs tbt tfa tfb tff tfs tft tsa tsb tsf tss tst tta ttb ttf tts ttt