|       /|\
    |        |
 in |        | out
    |        |
   \|/       | 
-------------------
|   external if   |
-------------------
    |        |    ----- 
    |   PF   |    | d |   in    
    |        #----| m |<---------
    |             | z |          
    #------->-----|   |--------->
                  | i |   out
                  | f |
                  ----- 
-------------------
|   internal if   |
-------------------
    |       /|\
    |        |
out |        | in
    |        |