a-a a-b aaa aab aaf aao aar aax ab- aba abb abf abo abr abx af- afa afb aff afo afr afx aoa aob aof aoo aor aox ar- ara arb arf aro arr arx ax- axa axo axr axx baa bab baf bao bar bax bba bbb bbf bbo bbr bbx bfa bfb bff bfo bfr bfx boa bob bof boo bor box bra brb brf bro brr brx bxb bxo bxx faa fab faf fao far fax fba fbb fbf fbo fbr fbx ffa fff ffo ffr ffx foa fob fof foo for fox fra frb frf fro frr fxr fxx oaa oab oaf oao oar oax ob- oba obb obf obo obr obx of- ofa ofb off ofo ofr ofx oo- ooa oob oof ooo oor oox or- ora orb orf oro orr orx ox- oxa oxb oxf oxo oxx ra- raa rab raf rao rar rax rba rbb rbf rbo rbr rbx rfa rfb rff rfo rfr roa rob rof roo ror rox rra rrb rrf rrr rxr xaa xaf xar xbr xbx xfa xfx xoa xof xoo xor xox xrf xrx xxo xxx