a-a a-b a-c aaa aab aac aaf aak aao ab- aba abb abc abf abk abo ac- aca acb acc acf ack aco af- afa afb afc aff afk afo aka akb akc akk ako aoa aob aoc aof aok aoo baa bab bac baf bak bao bba bbb bbc bbf bbk bbo bca bcb bcc bcf bck bco bfa bfb bfc bff bfo bka bkb bkc bkf bkk bko boa bob boc bof bok boo c-- caa cab cac caf cak cao cba cbb cbc cbf cbk cbo cca ccb ccc ccf cck cco cfa cfb cfc cff cfk cfo cka ckb ckc cko co- coa cob coc cof cok coo faa fab fac faf fak fao fba fbb fbc fbf fbk fbo fca fcb fcc fcf fck fco ffa ffc fff ffk ffo fka fkb fkc fkf fkk foa fob foc fof fok foo ka- kaa kab kac kaf kak kao kba kbb kbc kbf kbk kbo kca kcb kcc kck kfa kfc kff kfo kkb kkc kkk koa kob koc kof kok koo oaa oab oac oaf oak oao ob- oba obb obc obf obk obo oc- oca ocb occ ocf ock oco of- ofa ofb ofc off ofo oka okb okc okk oko oo- ooa oob ooc oof ook ooo