-er eee eef eer ees eet eex ef- efe eff efr efs eft efx ere erf err ers ert es- ese esf esr ess est esx ete etf etr ets ett etx ex- exe exr exs ext exx fee fef fer fes fet fex ffe fff ffr ffs fft ffx fre frf frr frs frt fse fsf fsr fss fst fsx fte ftf ftr fts ftt ftx fxe fxr fxs fxt fxx re- ree ref rer res ret rex rfe rff rfr rfs rft rrf rrr rrs rrt rse rsf rsr rss rst rsx rte rtf rtr rts rtt rtx rxr rxs se- see sef ser ses set sex sfe sff sfr sfs sft sfx sr- sre srf srr srs srt srx ss- sse ssf ssr sss sst ssx ste stf str sts stt stx sxs sxt t-x te- tee tef ter tes tet tex tfe tff tfr tfs tft tfx tre trf trr trs trt trx tse tsf tsr tss tst tsx tte ttf ttr tts ttt ttx txe txf txs txt xee xer xes xet xex xfe xfs xft xfx xrf xrt xrx xss xst xtf xtr xts xtx xxx