-er bbb bbc bbe bbo bbr bcb bcc bce bco bcr be- beb bec bee beo ber bob boc boe boo bor brb bre bro brr c-- cbb cbc cbe cbo cbr ccb ccc cce cco ccr ce- ceb cec cee ceo cer co- cob coc coe coo cor crb crc cre cro crr ebb ebc ebe ebo ebr ec- ecb ecc ece eco ecr eeb eec eee eeo eer eo- eob eoc eoe eoo eor erb erc ere ero err ob- obb obc obe obo obr oc- ocb occ oco ocr oeb oec oee oeo oer oo- oob ooc ooo oor or- orb orc ore oro orr rbb rbc rbe rbo rbr rcb rcc rce rco rcr re- reb rec ree reo rer rob roc roe roo ror rrb rrc rrr