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