a-a a-b aaa aab aao aar aav ab- aba abb abo abr abv aoa aob aoo aor aov ar- ara arb aro arr arv av- ava avb avo avr baa bab bao bar bav bba bbb bbo bbr boa bob boo bor bov bra brb bro brr brv bva bvb bvo bvr bvv oaa oab oao oar oav ob- oba obb obo obr obv oo- ooa oob ooo oor oov or- ora orb oro orr orv ov- ova ovb ovo ovv ra- raa rab rao rar rav rba rbb rbo rbr roa rob roo ror rov rra rrb rrr rva rvb rvo rvr rvv v-a vaa vab vao var vav vba vbo vbr vbv voa vob voo vor vov vra vrb vrr vva vvb vvo vvr vvv