graph TD
A -->|forward| B
B -->|back| A
---
+---------+      
|         |      
|    A    |<--+  
|         |   |  
+---------+   |  
     |        |  
     |        |  
  forward   back 
     |        |  
     v        |  
+---------+   |  
|         |   |  
|    B    |---+  
|         |      
+---------+      