a-a a-b aaa aab aaf aan aao ab- aba abb abf abn abo af- afa afb aff afn afo an- ana anb anf ann ano aoa aob aof aon aoo baa bab baf ban bao bba bbb bbf bbn bbo bfa bfb bff bfn bfo bna bnb bnf bnn bno boa bob bof bon boo faa fab faf fan fao fba fbb fbf fbn fbo ffa fff ffn ffo fna fnb fnf fnn foa fob fof fon foo naa nab naf nan nao nba nbb nbf nbn nbo nfa nfb nff nfn nfo nna nnb nnf nnn nno no- noa nob nof non noo oaa oab oaf oan oao ob- oba obb obf obn obo of- ofa ofb off ofn ofo on- ona onb onf onn ono oo- ooa oob oof oon ooo