Name
fence_later — 
     return the chronologically later fence
 
Arguments
- f1
- 
     [in]	the first fence from the same context
     
- f2
- 
     [in]	the second fence from the same context
     
Description
   Returns NULL if both fences are signaled, otherwise the fence that would be
   signaled last. Both fences must be from the same context, since a seqno is
   not re-used across contexts.