-er a-a a-b aaa aab aae aak aao aar aax ab- aba abb abe abk abo abr abx aea aeb aee aek aeo aer aex aka akb ake akk ako akr aoa aob aoe aok aoo aor aox ar- ara arb are ark aro arr arx ax- axa axe axk axo axr axx baa bab bae bak bao bar bax bba bbb bbe bbk bbo bbr bbx be- bea beb bee bek beo ber bex bka bkb bke bkk bko bkr bkx boa bob boe bok boo bor box bra brb bre brk bro brr brx bxb bxo bxx eaa eab eae eak eao ear eax eba ebb ebe ebk ebo ebr ebx eea eeb eee eek eeo eer eex eka ekb eke eko ekr eo- eoa eob eoe eok eoo eor era erb ere erk ero err ex- exa exe exo exr exx ka- kaa kab kae kak kao kar kax kba kbb kbe kbk kbo kbr kbx ke- kea keb kee kek keo ker kex kkb kke kkk kkr koa kob koe kok koo kor kra krk kro krr kxo oaa oab oae oak oao oar oax ob- oba obb obe obk obo obr obx oea oeb oee oeo oer oex oka okb oke okk oko okr oo- ooa oob ook ooo oor oox or- ora orb ore ork oro orr orx ox- oxa oxb oxo oxx ra- raa rab rae rak rao rar rax rba rbb rbe rbk rbo rbr rbx re- rea reb ree rek reo rer rex rkb rke rkk rko roa rob roe rok roo ror rox rra rrb rrk rrr rxr xaa xak xar xbe xbr xbx xee xer xex xkb xke xoa xok xoo xor xox xrx xxo xxx