c-- ccc ccf cck cco ccr ccs cfc cff cfk cfo cfr cfs ckc cko cks co- coc cof cok coo cor cos crc crf crk cro crr crs csc csf csk cso csr css fcc fcf fck fco fcr fcs ffc fff ffk ffo ffr ffs fkc fkf fkk foc fof fok foo for fos frc frf frk fro frr frs fsc fsf fsk fso fsr fss kcc kck kcr kcs kfc kff kfo kfs kkc kkk kkr kks koc kof kok koo kor kos krc krf krk kro krr krs ksc ksf ksk kso ksr kss oc- occ ocf ock oco ocr ocs of- ofc off ofo ofr ofs okc okk oko okr oks oo- ooc oof ook ooo oor oos or- orc orf ork oro orr ors os- osc osf osk oso osr oss rcc rcf rco rcr rcs rfc rff rfk rfo rfr rfs rkc rkf rkk rko rks roc rof rok roo ror ros rrc rrf rrk rrr rrs rsc rsf rsk rso rsr rss scc scf sck sco scr scs sfc sff sfk sfo sfr sfs sk- skc skf skk sko skr sks soc sof sok soo sor sos sr- src srf srk sro srr srs ss- ssc ssf ssk sso ssr sss