if ($a) {
        if ($b) {
            &S1;
        } else {
            &S2;
        }
    } else {
        if ($c) {
            &S3;
        } else {
            &S4;
        }
    }

##</code><code>##

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

##</code><code>##

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

##</code><code>##

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