b-u bbb bbr bbs bbt bbu brb brr brs brt bru bsb bsr bss bst bsu btb btr bts btt btu bub bur bus but buu rbb rbr rbs rbt rbu rrb rrr rrs rrt rru rsb rsr rss rst rsu rtb rtr rts rtt rtu rub rur rus rut ruu sbb sbr sbs sbt sbu sr- srb srr srs srt ss- ssb ssr sss sst ssu stb str sts stt stu sub sur sus sut suu tbb tbr tbs tbt tbu trb trr trs trt tru tsb tsr tss tst tsu ttb ttr tts ttt ttu tub tur tus tut tuu ubb ubr ubs ubt ubu ur- urb urr urs urt uru usb usr uss ust usu utb utr uts utt utu uur uus uut uuu