a-a a-b a-i aaa aab aai aao aas ab- aba abb abi abo abs ai- aia aib aii aio ais aoa aob aoi aoo aos as- asa asb asi aso ass baa bab bai bao bas bba bbb bbi bbo bbs bi- bia bib bii bio bis boa bob boi boo bos bsa bsb bsi bso bss ia- iaa iab iai iao ias iba ibb ibi ibo ibs iia iib iii iio iis io- ioa iob ioi ioo ios is- isa isb isi iso iss o-i oaa oab oai oao oas ob- oba obb obi obo obs oia oib oii oio ois oo- ooa oob ooi ooo oos os- osa osb osi oso oss saa sab sai sao sas sba sbb sbi sbo sbs sia sib sii sio sis soa sob soi soo sos ss- ssa ssb ssi sso sss