a-a a-b a-c aaa aab aac aad aao ab- aba abb abc abd abo ac- aca acb acc acd aco ad- ada adb adc add ado aoa aob aoc aod aoo baa bab bac bad bao bba bbb bbc bbd bbo bca bcb bcc bcd bco bda bdb bdc bdd bdo boa bob boc bod boo c-- caa cab cac cad cao cba cbb cbc cbd cbo cca ccb ccc ccd cco cda cdb cdc cdd cdo co- coa cob coc cod coo daa dab dac dad dao dba dbb dbc dbd dbo dca dcb dcc dcd dco dda ddb ddc ddd ddo doa dob doc dod doo oaa oab oac oad oao ob- oba obb obc obd obo oc- oca ocb occ ocd oco oda odb odc odd odo oo- ooa oob ooc ood ooo