+-----(b)->[S1]--+ | | +-----(a)->[test b]--+ |--+ | | | | | +-(not-b)->[S2]--+ | [test a]--| |-->[end] | +-----(c)->[S3]--+ | | | | | +-(not-a)->[test c]--+ |--+ | | +-(not-c)->[S4]--+