a-a a-b a-t aaa aab aao aar aat ab- aba abb abo abr abt aoa aob aoo aor aot ar- ara arb aro arr art at- ata atb ato atr att baa bab bao bar bat bba bbb bbo bbr bbt boa bob boo bor bot bra brb bro brr brt bta btb bto btr btt oaa oab oao oar oat ob- oba obb obo obr obt oo- ooa oob ooo oor oot or- ora orb oro orr ort ot- ota otb oto otr ott ra- raa rab rao rar rat rba rbb rbo rbr rbt roa rob roo ror rot rra rrb rrr rrt rta rtb rto rtr rtt taa tab tao tar tat tba tbb tbo tbr tbt to- toa tob too tor tot tra trb tro trr trt tta ttb tto ttr ttt