I thought about it.  The issue its actually pretty challenging to use for my purpose.  I would have to nest another function call and then define a "null" V to detect whether the input to the mergeFunction was the original value or the default value.  The semantics I used for update are:

```
if( k exists ) then V[k] = mergeF(V[k], newV)
else V[k] = newV
```