+---+                                                        +---+
|S1 |====== \               Forward -->             / =======|R1 |
+---+       \\                                     //        +---+
             \\                                   //
+---+       +-----+                            +-----+       +---+
|S2 |=======|  A  |--------------------------->|  B  |=======|R2 |
+---+       |     |<---------------------------|     |       +---+
            +-----+                            +-----+
(...)         //                                  \\         (...)
             //          <-- Backward              \\
+---+       //                                      \\       +---+
|Sn |====== /                                        \ ======|Rn |
+---+                                                        +---+