graph LR
A & B --> C & D
---
+---+     +---+
|   |     |   |
| A |--+->| C |
|   |  |  |   |
+---+  |  +---+
  |    |       
  |    |       
  +----+       
  |    |       
  |    |       
+---+  |  +---+
|   |  |  |   |
| B |--+->| D |
|   |     |   |
+---+     +---+
