-er bbb bbc bbe bbk bbo bbr bbs bcb bcc bce bck bco bcr bcs be- beb bec bee bek beo ber bes bkb bkc bke bkk bko bkr bks bob boc boe bok boo bor bos brb bre brk bro brr brs bsb bsc bse bsk bso bsr bss c-- cbb cbc cbe cbk cbo cbr cbs ccb ccc cce cck cco ccr ccs ce- ceb cec cee cek ceo cer ces ckb ckc cko cks co- cob coc coe cok coo cor cos crb crc cre crk cro crr crs csb csc cse csk cso csr css ebb ebc ebe ebk ebo ebr ebs ec- ecb ecc ece eck eco ecr ecs eeb eec eee eek eeo eer ees ekb ekc eke eko ekr eks eo- eob eoc eoe eok eoo eor eos erb erc ere erk ero err ers es- esb esc ese esk eso esr ess kbb kbc kbe kbk kbo kbr kbs kcb kcc kce kck kcr kcs ke- keb kec kee kek keo ker kes kkb kkc kke kkk kkr kks kob koc koe kok koo kor kos krc krk kro krr krs ksb ksc kse ksk kso ksr kss ob- obb obc obe obk obo obr obs oc- ocb occ ock oco ocr ocs oeb oec oee oeo oer oes okb okc oke okk oko okr oks oo- oob ooc ook ooo oor oos or- orb orc ore ork oro orr ors os- osb osc ose osk oso osr oss rbb rbc rbe rbk rbo rbr rbs rcb rcc rce rco rcr rcs re- reb rec ree rek reo rer res rkb rkc rke rkk rko rks rob roc roe rok roo ror ros rrb rrc rrk rrr rrs rsb rsc rse rsk rso rsr rss sbb sbc sbe sbk sbo sbr sbs scb scc sce sck sco scr scs se- seb sec see sek seo ser ses sk- skb skc ske skk sko skr sks sob soc soe sok soo sor sos sr- srb src sre srk sro srr srs ss- ssb ssc sse ssk sso ssr sss