a-a a-b aaa aab aao aas aax ab- aba abb abo abs abx aoa aob aoo aos aox as- asa asb aso ass asx ax- axa axo axs axx baa bab bao bas bax bba bbb bbo bbs bbx boa bob boo bos box bsa bsb bso bss bsx bxb bxo bxx oaa oab oao oas oax ob- oba obb obo obs obx oo- ooa oob ooo oos oox os- osa osb oso oss osx ox- oxa oxb oxo oxx saa sab sao sas sax sba sbb sbo sbs sbx soa sob soo sos sox ss- ssa ssb sso sss ssx sxs xaa xas xbx xoa xoo xos xox xsa xsb xss xxo xxx