a-a a-b a-c aaa aab aac aak aan aao ab- aba abb abc abk abn abo ac- aca acb acc ack acn aco aka akb akc akk akn ako an- ana anb anc ank ann ano aoa aob aoc aok aon aoo baa bab bac bak ban bao bba bbb bbc bbk bbn bbo bca bcb bcc bck bcn bco bka bkb bkc bkk bkn bko bna bnb bnc bnk bnn bno boa bob boc bok bon boo c-- caa cab cac cak can cao cba cbb cbc cbk cbn cbo cca ccb ccc cck ccn cco cka ckb ckc cko cn- cna cnb cnc cnn cno co- coa cob coc cok con coo ka- kaa kab kac kak kan kao kba kbb kbc kbk kbn kbo kca kcb kcc kck kcn kkb kkc kkk kkn kn- kna knc knk knn kno koa kob koc kok kon koo naa nab nac nak nan nao nba nbb nbc nbk nbn nbo nca ncb ncc nck ncn nco nka nkc nkk nkn nko nna nnb nnc nnk nnn nno no- noa nob noc nok non noo oaa oab oac oak oan oao ob- oba obb obc obk obn obo oc- oca ocb occ ock ocn oco oka okb okc okk oko on- ona onb onc onk onn ono oo- ooa oob ooc ook oon ooo