a-a a-b aaa aab aaf aag aao ab- aba abb abf abg abo af- afa afb aff afg afo ag- aga agb agf agg ago aoa aob aof aog aoo baa bab baf bag bao bba bbb bbf bbg bbo bfa bfb bff bfg bfo bga bgb bgf bgg bgo boa bob bof bog boo faa fab faf fag fao fba fbb fbf fbg fbo ffa fff ffg ffo fga fgb fgf fgg foa fob fof fog foo gaa gab gaf gag gao gba gbf gbg gbo gfa gfb gff gfo gga ggb ggg ggo goa gob gof gog goo oaa oab oaf oag oao ob- oba obb obf obg obo of- ofa ofb off ofg ofo oga ogf ogg ogo oo- ooa oob oof oog ooo