a-a a-b aaa aab aaf aak aao ab- aba abb abf abk abo af- afa afb aff afk afo aka akb akk ako aoa aob aof aok aoo baa bab baf bak bao bba bbb bbf bbk bbo bfa bfb bff bfo bka bkb bkf bkk bko boa bob bof bok boo faa fab faf fak fao fba fbb fbf fbk fbo ffa fff ffk ffo fka fkb fkf fkk foa fob fof fok foo ka- kaa kab kaf kak kao kba kbb kbf kbk kbo kfa kff kfo kkb kkk koa kob kof kok koo oaa oab oaf oak oao ob- oba obb obf obk obo of- ofa ofb off ofo oka okb okk oko oo- ooa oob oof ook ooo