+-----(b)->[S1]->[S5]-+
                               |                     |
          +-----(a)->[test b]--+                     |--+
          |                    |                     |  |
          |                    +-(not-b)->[S2]->[S6]-+  |
[test a]--|                                             |-->[end]
          |                    +-----(c)->[S3]->[S5]-+  |
          |                    |                     |  |
          +-(not-a)->[test c]--+                     |--+
                               |                     |
                               +-(not-c)->[S4]->[S6]-+