a-a a-b aaa aab aao aar aax ab- aba abb abo abr abx aoa aob aoo aor aox ar- ara arb aro arr arx ax- axa axo axr axx baa bab bao bar bax bba bbb bbo bbr bbx boa bob boo bor box bra brb bro brr brx bxb bxo bxx oaa oab oao oar oax ob- oba obb obo obr obx oo- ooa oob ooo oor oox or- ora orb oro orr orx ox- oxa oxb oxo oxx ra- raa rab rao rar rax rba rbb rbo rbr rbx roa rob roo ror rox rra rrb rrr rxr xaa xar xbr xbx xoa xoo xor xox xrx xxo xxx