a-a a-b a-i aaa aab aaf aai aas ab- aba abb abf abi abs af- afa afb aff afi afs ai- aia aib aif aii ais as- asa asb asf asi ass baa bab baf bai bas bba bbb bbf bbi bbs bfa bfb bff bfi bfs bi- bia bib bif bii bis bsa bsb bsf bsi bss faa fab faf fai fas fba fbb fbf fbi fbs ffa fff ffi ffs fia fib fif fii fis fsa fsb fsf fsi fss i-f ia- iaa iab iaf iai ias iba ibb ibf ibi ibs ifa ifb iff ifi ifs iia iib iif iii iis is- isa isb isf isi iss saa sab saf sai sas sba sbb sbf sbi sbs sfa sfb sff sfi sfs sia sib sif sii sis ss- ssa ssb ssf ssi sss