a-a a-b a-c aaa aab aac aaf aak aas aax ab- aba abb abc abf abk abs abx ac- aca acb acc acf ack acs acx af- afa afb afc aff afk afs afx aka akb akc akk aks as- asa asb asc asf ask ass asx ax- axa axk axs axx baa bab bac baf bak bas bax bba bbb bbc bbf bbk bbs bbx bca bcb bcc bcf bck bcs bcx bfa bfb bfc bff bfs bfx bka bkb bkc bkf bkk bks bkx bsa bsb bsc bsf bsk bss bsx bxb bxx c-- caa cab cac caf cak cas cax cba cbb cbc cbf cbk cbs cbx cca ccb ccc ccf cck ccs ccx cfa cfb cfc cff cfk cfs cfx cka ckb ckc cks ckx csa csb csc csf csk css csx cxc cxf cxk cxs cxx faa fab fac faf fak fas fax fba fbb fbc fbf fbk fbs fbx fca fcb fcc fcf fck fcs fcx ffa ffc fff ffk ffs ffx fka fkb fkc fkf fkk fsa fsb fsc fsf fsk fss fsx fxs fxx ka- kaa kab kac kaf kak kas kax kba kbb kbc kbf kbk kbs kbx kca kcb kcc kck kcs kcx kfa kfc kff kfs kfx kkb kkc kkk kks ksa ksb ksc ksf ksk kss saa sab sac saf sak sas sax sba sbb sbc sbf sbk sbs sbx sca scb scc scf sck scs scx sfa sfb sfc sff sfk sfs sfx sk- ska skb skc skf skk sks skx ss- ssa ssb ssc ssf ssk sss ssx sxc sxs xaa xac xaf xak xas xbc xbx xcb xcf xcx xfa xfs xfx xkb xks xsa xsb xsc xss xxx