a-a a-c aaa aac aaf aao aas ac- aca acc acf aco acs af- afa afc aff afo afs aoa aoc aof aoo aos as- asa asc asf aso ass c-- caa cac caf cao cas cca ccc ccf cco ccs cfa cfc cff cfo cfs co- coa coc cof coo cos csa csc csf cso css faa fac faf fao fas fca fcc fcf fco fcs ffa ffc fff ffo ffs foa foc fof foo fos fsa fsc fsf fso fss oaa oac oaf oao oas oc- oca occ ocf oco ocs of- ofa ofc off ofo ofs oo- ooa ooc oof ooo oos os- osa osc osf oso oss saa sac saf sao sas sca scc scf sco scs sfa sfc sff sfo sfs soa soc sof soo sos ss- ssa ssc ssf sso sss