a-a a-b aaa aab aad aaf aar ab- aba abb abd abf abr ad- ada adb add adf adr af- afa afb afd aff afr ar- ara arb ard arf arr baa bab bad baf bar bba bbb bbd bbf bbr bda bdb bdd bdf bdr bfa bfb bfd bff bfr bra brb brd brf brr daa dab dad daf dar dba dbb dbd dbf dbr dda ddb ddd ddf ddr dfa dfb dfd dff dfr dra drb drd drr faa fab fad faf far fba fbb fbd fbf fbr fda fdb fdd fdf fdr ffa ffd fff ffr fra frb frd frf frr ra- raa rab rad raf rar rba rbb rbd rbf rbr rda rdb rdd rdf rdr rfa rfb rfd rff rfr rra rrb rrd rrf rrr