+------------+                   +------------+
                |     D1     |                   |     D2     |
       1-0-1-1 /-\          /-\                 /-\          /-\
    <--------->| | 1-0-1    | |---------------->| | 2-1-1    | |
       1-0-1-2 | |    1-2-1 | |<----------------| |    2-0-1 | |
    <--------> \-/  1-3-1   \-/                 \-/  2-3-1   \-/
                |   /----\   |                   |   /----\   |
                +---|    |---+                   +---|    |---+
                    \----/                           \----/
                     A  |                             A  |
                     |  |                             |  |
                     |  |                             |  |
                     |  |       +------------+        |  |
                     |  |       |     D3     |        |  |
                     |  |      /-\          /-\       |  |
                     |  +----->| | 3-1-1    | |-------+  |
                     +---------| |    3-2-1 | |<---------+
                               \-/          \-/
                                |            |
                                +------------+