a-a a-b aaa aab aaf aan aar ab- aba abb abf abn abr af- afa afb aff afn afr an- ana anb anf ann anr ar- ara arb arf arn arr baa bab baf ban bar bba bbb bbf bbn bbr bfa bfb bff bfn bfr bna bnb bnf bnn bnr bra brb brf brn brr faa fab faf fan far fba fbb fbf fbn fbr ffa fff ffn ffr fna fnb fnf fnn fnr fra frb frf frn frr naa nab naf nan nar nba nbb nbf nbn nbr nfa nfb nff nfn nfr nna nnb nnf nnn nra nrb nrf nrn nrr ra- raa rab raf ran rar rba rbb rbf rbn rbr rfa rfb rff rfn rfr rna rnb rnn rnr rra rrb rrf rrn rrr