-er a-a a-b a-t aaa aab aae aar aat ab- aba abb abe abr abt aea aeb aee aer aet ar- ara arb are arr art at- ata atb ate atr att baa bab bae bar bat bba bbb bbe bbr bbt be- bea beb bee ber bet bra brb bre brr brt bta btb bte btr btt eaa eab eae ear eat eba ebb ebe ebr ebt eea eeb eee eer eet era erb ere err ert eta etb ete etr ett ra- raa rab rae rar rat rba rbb rbe rbr rbt re- rea reb ree rer ret rra rrb rrr rrt rta rtb rte rtr rtt taa tab tae tar tat tba tbb tbe tbr tbt te- tea teb tee ter tet tra trb tre trr trt tta ttb tte ttr ttt