a-a a-b aaa aab aar aas aax ab- aba abb abr abs abx ar- ara arb arr ars arx as- asa asb asr ass asx ax- axa axr axs axx baa bab bar bas bax bba bbb bbr bbs bbx bra brb brr brs brx bsa bsb bsr bss bsx bxb bxx ra- raa rab rar ras rax rba rbb rbr rbs rbx rra rrb rrr rrs rsa rsb rsr rss rsx rxr rxs saa sab sar sas sax sba sbb sbr sbs sbx sr- sra srb srr srs srx ss- ssa ssb ssr sss ssx sxs xaa xar xas xbr xbx xrx xsa xsb xss xxx