eee eef ees eet eex ef- efe eff efs eft efx es- ese esf ess est esx ete etf ets ett etx ex- exe exs ext exx fee fef fes fet fex ffe fff ffs fft ffx fse fsf fss fst fsx fte ftf fts ftt ftx fxe fxs fxt fxx se- see sef ses set sex sfe sff sfs sft sfx ss- sse ssf sss sst ssx ste stf sts stt stx sxs sxt t-x te- tee tef tes tet tex tfe tff tfs tft tfx tse tsf tss tst tsx tte ttf tts ttt ttx txe txf txs txt xee xes xet xex xfe xfs xft xfx xss xst xtf xts xtx xxx