a-a a-b a-c aaa aab aac aak aar ab- aba abb abc abk abr ac- aca acb acc ack acr aka akb akc akk akr ar- ara arb arc ark arr baa bab bac bak bar bba bbb bbc bbk bbr bca bcb bcc bck bcr bka bkb bkc bkk bkr bra brb brk brr c-- caa cab cac cak car cba cbb cbc cbk cbr cca ccb ccc cck ccr cka ckb ckc cra crb crc crk crr ka- kaa kab kac kak kar kba kbb kbc kbk kbr kca kcb kcc kck kcr kkb kkc kkk kkr kra krc krk krr ra- raa rab rac rak rar rba rbb rbc rbk rbr rca rcb rcc rcr rkb rkc rkk rra rrb rrc rrk rrr