a-a a-b aaa aab aak aao aar aax ab- aba abb abk abo abr abx aka akb akk ako akr aoa aob aok aoo aor aox ar- ara arb ark aro arr arx ax- axa axk axo axr axx baa bab bak bao bar bax bba bbb bbk bbo bbr bbx bka bkb bkk bko bkr bkx boa bob bok boo bor box bra brb brk bro brr brx bxb bxo bxx ka- kaa kab kak kao kar kax kba kbb kbk kbo kbr kbx kkb kkk kkr koa kob kok koo kor kra krk kro krr kxo oaa oab oak oao oar oax ob- oba obb obk obo obr obx oka okb okk oko okr oo- ooa oob ook ooo oor oox or- ora orb ork oro orr orx ox- oxa oxb oxo oxx ra- raa rab rak rao rar rax rba rbb rbk rbo rbr rbx rkb rkk rko roa rob rok roo ror rox rra rrb rrk rrr rxr xaa xak xar xbr xbx xkb xoa xok xoo xor xox xrx xxo xxx