+-------+     +-------+     +-------+
     |   a   |---->|   s   |---->|   c   |
     +-------+     +-------+     +-------+
                  +---------+             
     +-------+   / +-------+ \   +-------+
     |   o   |--+  |   -   |  +->|   o   |
     +-------+     +-------+     +-------+