a-a a-b a-t aaa aab aar aat aax ab- aba abb abr abt abx ar- ara arb arr art arx at- ata atb atr att atx ax- axa axr axt axx baa bab bar bat bax bba bbb bbr bbt bbx bra brb brr brt brx bta btb btr btt btx bxb bxx ra- raa rab rar rat rax rba rbb rbr rbt rbx rra rrb rrr rrt rta rtb rtr rtt rtx rxr t-x taa tab tar tat tax tba tbb tbr tbt tra trb trr trt trx tta ttb ttr ttt ttx txt xaa xar xat xbr xbx xrt xrx xtr xtx xxx