a-a a-b aaa aab aaf aak aar ab- aba abb abf abk abr af- afa afb aff afk afr aka akb akk akr ar- ara arb arf ark arr baa bab baf bak bar bba bbb bbf bbk bbr bfa bfb bff bfr bka bkb bkf bkk bkr bra brb brf brk brr faa fab faf fak far fba fbb fbf fbk fbr ffa fff ffk ffr fka fkb fkf fkk fra frb frf frk frr ka- kaa kab kaf kak kar kba kbb kbf kbk kbr kfa kff kkb kkk kkr kra krf krk krr ra- raa rab raf rak rar rba rbb rbf rbk rbr rfa rfb rff rfk rfr rkb rkf rkk rra rrb rrf rrk rrr