a-a a-b aaa aab aaf aag aan ab- aba abb abf abg abn af- afa afb aff afg afn ag- aga agb agf agg agn an- ana anb anf ang ann baa bab baf bag ban bba bbb bbf bbg bbn bfa bfb bff bfg bfn bga bgb bgf bgg bgn bna bnb bnf bng bnn faa fab faf fag fan fba fbb fbf fbg fbn ffa fff ffg ffn fga fgb fgf fgg fgn fna fnb fnf fng fnn gaa gab gaf gag gan gba gbf gbg gbn gfa gfb gff gga ggb ggg ggn gna gnb gnf gnn naa nab naf nag nan nba nbb nbf nbg nbn nfa nfb nff nfn nga ngb ngf ngg ngn nna nnb nnf nng nnn