a-a a-b aaa aab aaf aao aay ab- aba abb abf abo aby af- afa afb aff afo afy aoa aob aof aoo ay- aya ayf ayo baa bab baf bao bay bba bbb bbf bbo bby bfa bfb bff bfo bfy boa bob bof boo boy by- bya byf byo faa fab faf fao fay fba fbb fbf fbo ffa fff ffo ffy foa fob fof foo foy fy- fya fyb fyo oaa oab oaf oao ob- oba obb obf obo oby of- ofa ofb off ofo oo- ooa oob oof ooo oya oyo yaa yab yaf yao yay ybf yby yfa yff yob yof yoo yoy yyb yyf yyy