a-a a-b a-c aaa aab aac aak aao ab- aba abb abc abk abo ac- aca acb acc ack aco aka akb akc akk ako aoa aob aoc aok aoo baa bab bac bak bao bba bbb bbc bbk bbo bca bcb bcc bck bco bka bkb bkc bkk bko boa bob boc bok boo c-- caa cab cac cak cao cba cbb cbc cbk cbo cca ccb ccc cck cco cka ckb ckc cko co- coa cob coc cok coo ka- kaa kab kac kak kao kba kbb kbc kbk kbo kca kcb kcc kck kkb kkc kkk koa kob koc kok koo oaa oab oac oak oao ob- oba obb obc obk obo oc- oca ocb occ ock oco oka okb okc okk oko oo- ooa oob ooc ook ooo