a-a a-b aaa aab aad aaf aao ab- aba abb abd abf abo ad- ada adb add adf ado af- afa afb afd aff afo aoa aob aod aof aoo baa bab bad baf bao bba bbb bbd bbf bbo bda bdb bdd bdf bdo bfa bfb bfd bff bfo boa bob bod bof boo daa dab dad daf dao dba dbb dbd dbf dbo dda ddb ddd ddf ddo dfa dfb dfd dff dfo doa dob dod dof doo faa fab fad faf fao fba fbb fbd fbf fbo fda fdb fdd fdf fdo ffa ffd fff ffo foa fob fof foo oaa oab oad oaf oao ob- oba obb obd obf obo oda odb odd odf odo of- ofa ofb ofd off ofo oo- ooa oob ood oof ooo