+---+                                                         +---+
|S1 |===== \                Forward -->              / =======|R1 |
+---+      \\                                       //        +---+
            \\                                     //
+---+       +-----+                               +-----+       +---+
|S2 |=======|  A  |------------------------------>|  B  |=======|R2 |
+---+       |     |<------------------------------|     |       +---+
            +-----+                               +-----+
            //          <-- Backward               \\
+---+      //                                       \\       +---+
|S3 |===== /                                         \ ======|R3 |
+---+                                                        +---+