bbb bbf bbo bbr bbt bfb bff bfo bfr bft bob bof boo bor bot brb brf bro brr brt btb btf bto btr btt fbb fbf fbo fbr fbt fff ffo ffr fft fob fof foo for fot frb frf fro frr frt ftb ftf fto ftr ftt ob- obb obf obo obr obt of- ofb off ofo ofr oft oo- oob oof ooo oor oot or- orb orf oro orr ort ot- otb otf oto otr ott rbb rbf rbo rbr rbt rfb rff rfo rfr rft rob rof roo ror rot rrb rrf rrr rrt rtb rtf rto rtr rtt tbb tbf tbo tbr tbt tfb tff tfo tfr tft to- tob tof too tor tot trb trf tro trr trt ttb ttf tto ttr ttt